i18n string cleanup

Changes:
* Remove translation strings from assessment APIs
* Updated logging in XBlock mixins
* Added more verbose logging in assessment APIs
* Cleaned up i18n strings in templates.

@srpearce There are some wording changes to the error messages, but nothing student-facing.
@gradyward This touches some of the messaging templates, but the changes are very minor.
@stephensanchez please review, especially the logging changes.