Missing required option doesn't throw

This is a proposed fix for: airlift#22

This approach introduces a ParseResult object which allows for the collection of multiple parse errors from the user input
and then provides this back to the caller to decide what to do. E.G. send the user the to the help dialog.

This keeps the existing API and fits with the desire to keep the parsing logic agnostic to the details of the HelpOption.

The existing pattern still fails fast with an exception:

```java
MyCommand command = SingleCommand.singleCommand(MyCommand.class).parse(args);
```

This fix introduces the following pattern from the caller's perspective:

```java
ParseResult parseResult = new ParseResult();
MyCommand command = SingleCommand.singleCommand(MyCommand.class).parse(args, parseResult);

if (parseResult.hasErrors()) {
     command.helpOption.showHelpWithErrors(parseResult.errors());
}
```

src/main/java/io/airlift/airline/ParseResult.java

@@ -0,0 +1,22 @@
+package io.airlift.airline;
+
+import com.google.common.collect.ImmutableList;
+
+import java.util.LinkedList;
+import java.util.List;
+
+public class ParseResult {
+    private List<ParseException> errors = new LinkedList<>();
+
+    public void addError(ParseException error) {
+        errors.add(error);
+    }
+
+    public boolean hasErrors() {
+        return errors.size() != 0;
+    }
+
+    public List<ParseException> getErrors() {
+        return ImmutableList.copyOf(errors);
+    }
+}

src/main/java/io/airlift/airline/SingleCommand.java

@@ -57,12 +57,31 @@ public C parse(String... args)
     }
 
     public C parse(Iterable<String> args)
+    {
+        ParseResult parseResult = new ParseResult();
+        C command = parse(parseResult, args);
+
+        // For backward compatibility we fail fast here. Given that exceptions aren't a great way to handle user errors
+        // since there can be multiple we may consider deprecating this approach.
+        if (parseResult.hasErrors()) {
+            throw parseResult.getErrors().get(0);
+        }
+
+        return command;
+    }
+
+    public C parse(ParseResult parseResult, String... args)
+    {
+        return parse(parseResult, ImmutableList.copyOf(args));
+    }
+
+    public C parse(ParseResult parseResult, Iterable<String> args)
     {
         checkNotNull(args, "args is null");
-        
+
         Parser parser = new Parser();
         ParseState state = parser.parseCommand(commandMetadata, args);
-        validate(state);
+        validate(state, parseResult);
 
         CommandMetadata command = state.getCommand();
 
@@ -75,35 +94,35 @@ public C parse(Iterable<String> args)
                 ImmutableMap.<Class<?>, Object>of(CommandMetadata.class, commandMetadata));
     }
 
-    private void validate(ParseState state)
+    private void validate(ParseState state, ParseResult parseResult)
     {
         CommandMetadata command = state.getCommand();
         if (command == null) {
             List<String> unparsedInput = state.getUnparsedInput();
             if (unparsedInput.isEmpty()) {
-                throw new ParseCommandMissingException();
+                parseResult.addError(new ParseCommandMissingException());
             }
             else {
-                throw new ParseCommandUnrecognizedException(unparsedInput);
+                parseResult.addError(new ParseCommandUnrecognizedException(unparsedInput));
             }
         }
 
         ArgumentsMetadata arguments = command.getArguments();
         if (state.getParsedArguments().isEmpty() && arguments != null && arguments.isRequired()) {
-            throw new ParseArgumentsMissingException(arguments.getTitle());
+            parseResult.addError(new ParseArgumentsMissingException(arguments.getTitle()));
         }
 
         if (!state.getUnparsedInput().isEmpty()) {
-            throw new ParseArgumentsUnexpectedException(state.getUnparsedInput());
+            parseResult.addError(new ParseArgumentsUnexpectedException(state.getUnparsedInput()));
         }
 
         if (state.getLocation() == Context.OPTION) {
-            throw new ParseOptionMissingValueException(state.getCurrentOption().getTitle());
+            parseResult.addError(new ParseOptionMissingValueException(state.getCurrentOption().getTitle()));
         }
 
         for (OptionMetadata option : command.getAllOptions()) {
             if (option.isRequired() && !state.getParsedOptions().containsKey(option)) {
-                throw new ParseOptionMissingException(option.getOptions().iterator().next());
+                parseResult.addError(new ParseOptionMissingException(option.getOptions().iterator().next()));
             }
         }
     }

src/test/java/io/airlift/airline/SingleCommandTest.java

@@ -42,13 +42,13 @@
 import org.testng.annotations.Test;
 
 import javax.inject.Inject;
-
 import java.math.BigDecimal;
 import java.util.Arrays;
 import java.util.List;
 
 import static com.google.common.base.Predicates.compose;
 import static com.google.common.base.Predicates.equalTo;
+import static com.google.common.base.Predicates.instanceOf;
 import static com.google.common.collect.Iterables.find;
 import static io.airlift.airline.SingleCommand.singleCommand;
 import static org.testng.Assert.assertTrue;
@@ -244,12 +244,27 @@ public void requiredMainParameters()
         singleCommand(ArgsRequired.class).parse();
     }
 
+    @Test
+    public void requiredMainParamertersWithParseResult() {
+        ParseResult result = new ParseResult();
+        singleCommand(ArgsRequired.class).parse(result);
+        assertParseResultHasErrorOfType(result, ParseException.class);
+    }
+
     @Test(expectedExceptions = ParseException.class, expectedExceptionsMessageRegExp = ".*option.*missing.*")
     public void requiredOptions()
     {
         singleCommand(OptionsRequired.class).parse();
     }
 
+    @Test
+    public void requiredOptionsWithParseResult()
+    {
+        ParseResult result = new ParseResult();
+        singleCommand(OptionsRequired.class).parse(result);
+        assertParseResultHasErrorOfType(result, ParseException.class);
+    }
+
     @Test
     public void ignoresOptionalOptions()
     {
@@ -388,4 +403,10 @@ public static class CommandTest
         public Boolean interactive = false;
 
     }
+
+    private void assertParseResultHasErrorOfType(ParseResult result, Class clazz) {
+        Assert.assertEquals(result.hasErrors(), true);
+        String errorMessage = String.format("Expected ParseResult to contain at least one instance of: '%s'", clazz);
+        Assert.assertNotNull(find(result.getErrors(), instanceOf(clazz)), errorMessage);
+    }
 }