Prompt if the student enters the negative of the correct answer

A problem that seems to crop up often in negative number problems is the student enters almost the correct answer, but with the wrong sign (e.g. #11845).

This change presents a message when this happens, similar to the one for unsimplified fractions.

I'm concerned this might not be the best way to fix the problem though. I was thinking perhaps exercises could provide their own "common wrong answers" with appropriate messages?

Also if I've made a mistake it could break every exercise since this in the logic for determining if an answer is correct or not...