Followup 8a4058584: use a single media source for rerendering images

The code asked all media sources for a rerender and used whichever one
came in first, which meant that it might potentially render the wrong
image, or reject the promise if one source served an error response
before a good response from another source arrived.

Also reject the promise if we can't find an image source in the response.

Change-Id: I6b516ad41b8a9e2abd440625bb76f3e1abb54520
(cherry picked from commit 7035c8de01738fa45003b46d70bd7fbe3b48f097)
Merge "Followup 8a4058584: use a single media source for rerendering images" into wmf/1.22wmf22