report_error refactoring

[Vinatorul]

This refactor will help me a lot to change get_matches_with to return Result.
Also it is good for same code deletions.
Only I'm not sure with module and enum name, so waiting for your suggestions before merge ;)

[kbknapp]

Looks like a good start! I'd re-name the error enum to something like ClapError (or similar??) though because ArgNames confused me at first. And there are also errors that may not be related to args, or down the road there could be. Then we can also implement std::error::Error

[Vinatorul]

there is a big problem with Error trait. I tried to do it, but for now reports using huge bunch of App code, it is unattainable from Display trait, which is must-have if you implement Error
The main problem is that create_usage can't be used without App instance.
I have some ideas how to solve this in non rust ideology, but it is non goal for us, I think.

Hm, I think I have good idea how to solve it. Just generate error message with usage in get_matches_with and handle it in get_matches_from or get_matches_from_safe

[Vinatorul]

Resolved

[kbknapp]

Very nice!