Add way to customise message if a typed command is invalid

[dualspiral]

This arose from discussion with @Faithcaio - basically the exception messages returned from commands that aren't parsed correctly is... bad.

Currently, if you type in a command and it doesn't parse correctly, it states that it is an unknown command. Plugin developers might want return a better message that might display the usage (as an example) if their command is called incorrectly.

I will likely want to add a way to attach a UnknownCommandHandler (name pending) on the Command.Builder object that gets the partial command context and command string - but I do need to think about this a little more.

[ImMorpheus]

@dualspiral Is this fixed by #2296 and ValueParameterModifier#modifyExceptionMessage ?

[dualspiral]

Not at all.

[reflqct]

I think this would be really useful