Clean up logging in peer and self assessment mixins

I was trying to debug an issue in Vagrant and discovered that, under certain circumstances, we were showing a client-side error message when the user tried to peer assess, but we weren't logging it anywhere.

While cleaning that up, I noticed a few other places where we were internationalizing log messages and displaying exceptions directly to users.  I've also added test cases for errors we weren't covering.

@stephensanchez Please review.

@srpearce There's a small user-facing change in the self-assessment error message -- instead of showing the exception on an error, this will now display the message "Your self assessment could not be submitted."  None of the other text changes are user-facing.