Check for error before outputting srcset urls

If $thumb returns a MediaTransformError object, then $thumb->getUrl()
will return false, which will result in srcset="1.5x, 2x".

Bug: 67081
Change-Id: Idb6bbff6995536d20dac67ce12592f68f6d1235a
Merge "Check for error before outputting srcset urls"