Should warn when setting --check with non-existing rules

[jhauberg]

Currently, running --check with a rule that doesn't exist or was spelled incorrectly cause it to just be ignored. For example, mistakenly going:

$ comply src/ --check=line_too_long

When the rule name was really line-too-long.

This can be hard to spot, and would be neat if a diagnostic was emitted. Something like Rule 'line_too_long' was not found. Did you mean 'line-too-long'? (That last part only for extra credit...).

This should also apply for --except. Diagnostic should probably make it clear where the error was discovered; e.g. in either --except or --check. Maybe Excepted rule 'line_too_long' was not found.