Properly handle incorrect revisions in the github get_change() implementation.

Review Request #5015 — Created Nov. 19, 2013 and submitted — Latest diff uploaded

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master

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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.