Caught DoesNotExist exceptions in WebAPI, now raises 404

Review Request #3040 — Created April 3, 2012 and discarded — Latest diff uploaded

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This addresses issue 2551.

Caught the DoesNotExist exceptions in the web API, and raised a HTTP 404 exception instead.
I tried to use the same language as in http://reviews.reviewboard.org/r/9999/
Testing done in Linux 3.2.13 with Chromium 18.0.

I visited local links that pointed to invalid review requests:
http://localhost:8080/api/review-requests/99/diffs/1/files/1/diff-comments/
http://localhost:8080/api/review-requests/999999999/screenshots/