Tests for LMS-1532
Clear out the cache xmodule_instance after an exception in XModule init

This guarantees that the ErrorModule will be created, instead of
potentially using a partially instatiated broken module. Fixes
[LMS-1532].
Merge pull request #1746 from cpennington/fix-lms-1532

Fix LMS-1532 (partially instantiated XModules after an exception)