Fix AssertionErrors that were raised when creation of an XModule failed

The ErrorDescriptor wasn't getting the appropriate xmodule_runtime set,
and thus immediately raising an AssertionError when we tried to retrieve
and _xmodule from it.

[LMS-1388[
Merge pull request #1561 from cpennington/hotfix-assertion-error

Fix AssertionErrors that were raised when creation of an XModule failed