Properly handle incorrect revisions in the github get_change() implementation.
The get_change() implementation in the GitHub hosting service was letting a

bare Exception through in its failure case, which I think only really happens

when GitHub's API is having problems. This meant that the API would return a

500 and we'd get a traceback in our email. I've changed it to catch Exception

and re-raise as SCMError, which is caught by ReviewRequestResource.create().

Ran unit tests.