`feedback` flag shouldn't apply for IE

[Balint-R]

If the feedback flag of a custom checker (e.g. of type coci or testlib) is set to false, then feedback won't be included in the error message in case of an IE. I don't see any cases where it is desirable not to have the additional information for debugging, so the feedback flag should only apply for feedback to the user.

[kiritofeng]

I implemented the feedback flag poorly; a better implementation would probably pass the feedback unconditionally to the ContribModule in addition to a should_show_feedback boolean that implements the current feedback flag.