Fixed bug when user votes w/o permission - now displays a friendly error message instead of failing.

Fixed bug when hinter module displays a hint, then is asked to display nothing.  (Used to not update in this case.)
Fixed bug: When student submits empty string, hinter returns a 500 error.

Added tests to catch said bug.

Fixed some pylint violations.
Added error logging for unparsable answers.
Merge branch 'felix/hinter2' of https://github.com/edx/edx-platform into felix/hinter2

Conflicts:
	common/lib/xmodule/xmodule/crowdsource_hinter.py