REF: Moved mkrf_conf error to errors lib

[venarius]

Hey 👋

Since this project has a .rubocop.yml file, I guessed that doing some rubocop refactoring wouldn't be bad. Thats what I did here. Also extracted the mkrf_conf errors to the lib/errors.rb

Together with #29 all rubocop warnings / errors should be fixed now 🙂

[matthewd]

Hi!

Thanks for this!

Could you possibly split the error change into a separate PR?

That one's an easy merge, whereas the rubocop fixes -- which would normally be even easier -- will probably create conflicts with #27, so I'd rather hold off on those until I've decided whether I can stomach Standard's formatting predilections.

[venarius]

Yes sure, I will convert this to be only for the error.

[venarius]

@matthewd just changed this commit :)

[venarius]

I fixed the typo, changed the command name to be more broad and also added a little test that checks for the NoGemfile error :)

[matthewd]

Great, thanks!

(Though the flaw in the pattern is that it was already working -- I haven't actually checked, but I'm 95% sure that test would pass with the typo present.)