Send useful error message when assignments on dipatch.py and canvas don't match

[ttimbers]

If an instructor removes an assignment from Canvas, but doesn't remove it from dispatch then a useful error message should be logged.

[trevorcampbell]

This is already in #50 -- I think while we are doing big initial coding stuff we should collect all these unit test cases in one issue to keep @daisymzhang sane. Once we are happy with an initial version, we can create separate issues for each new one that pops up!