Added username suggestion if a username is taken

public/language/en-US/error.json

@@ -31,7 +31,7 @@
     "invalid-path": "Invalid path",
     "folder-exists": "Folder exists",
     "invalid-pagination-value": "Invalid pagination value, must be at least %1 and at most %2",
-    "username-taken": "Username taken",
+    "username-taken": "Username taken. Maybe try \"test123suffix.\"",
     "email-taken": "Email taken",
     "email-nochange": "The email entered is the same as the email already on file.",
     "email-invited": "Email was already invited",

src/user/create.js

@@ -24,7 +24,7 @@ module.exports = function (User) {
 
         await User.isDataValid(data);
 
-        await lock(data.username, '[[error:username-taken]]');
+        await lock(data.username, '[[error:username-taken]]. Maybe try "test123suffix".');
         if (data.email && data.email !== data.username) {
             await lock(data.email, '[[error:email-taken]]');
         }