Remove deprecated Exception.message calls; fix TIM-497

`Exception.message` is deprecated in Python 2.6, and the Ubuntu Python interpreter complains bitterly about this when running the test suite.

Also fixes [TIM-497](https://edx-wiki.atlassian.net/browse/TIM-497): Some error messages have u"" around them

@srpearce Please review the error message text in apps/openassessment/xblock/peer_assessment_mixin.py (line 81)