Fix exception being thrown on URLs with nonexistent review id

URLs like /api/review-requests/<id>/screenshots, where <id> is nonexistent,
until now threw an exception instead of translating to a proper 404. This change
catches the possible ObjectDoesNotExist of get_queryset() in
WebAPIResource.get_list.

Testing done:
webapi.tests.ScreenshotResourceTests.test_get_screenshots_with_invalid_review_request_id
in the corresponding change for revieboard tests this behavior.

Fixes bug 2551.
Reviewed at http://reviews.reviewboard.org/r/3767/