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