Fix unit tests on master due to the apiGet rename.

release-2.0.x renamed apiGet to api_get, but the new tests on master
weren't updated. This updates them to fix all tests.
Fix a unit test breakage in datagrids on master.

The rename of getContextVar to _get_context_var on release-2.0.x broke a
newer unit test on master. This updates the function call to use the new
name.
Fix AttributeError when trying to accessing invalid diff fragment.

When browsing to an invalid FileDiff via, we would raise a 404 exception but
this was getting caught prematurely and eventually causing a server error. This
change simply passes Http404 errors back up the stack.

Testing Done:
Opened an invalid fragment URL and saw a 404 page instead of a traceback.

Bugs closed: 3473

Reviewed at https://reviews.reviewboard.org/r/6089/
Merge branch 'release-2.0.x'