[CLI] Add graceful error handling for unknown command

[grokify]

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Description of the PR

Currently, the CLI throws an exception when an unknown command is encountered such as the following for --help as mentioned here: #490

It's hard to figure out the command line options when the software crashes when called with --help.

The uncaught exception will result in the following:

$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar --help
Exception in thread "main" io.airlift.airline.ParseArgumentsUnexpectedException: Found unexpected parameters: [--help]
	at io.airlift.airline.Cli.validate(Cli.java:148)
	at io.airlift.airline.Cli.parse(Cli.java:116)
	at io.airlift.airline.Cli.parse(Cli.java:97)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:58)

A few people have run into this as mentioned here: airlift/airline#45

--help is the first thing I run on a new command

This PR will return a graceful error for any unknown command by gracefully catching io.airlift.airline.ParseArgumentsUnexpectedException such as the following:

$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar --help
[error] Found unexpected parameters: [--help]

See 'openapi-generator-cli help' for usage.

A DWIM approach would alias --help to help but this may lead to more issues as we may need to alias all the other commands as well to avoid confusion. This approach returns an nicer error message and encourages the user to learn the correct syntax for the CLI. However, if we decide it is better to alias, I can look into this too.

[wing328]

Agreed that aliasing may result in a lot of works and your solution is much better than showing the exception.