Avoid attempting to prerender thumbnails that will fail

Avoid attempting to prerender thumbnails that will fail

For non-vectorial content, requesting a thumbnail larger than
the original results in a 500. Prerendering in its current form
introduces an increase in 500s that dilutes the real problematic
500s, making troubleshooting harder than it needs to be.

Change-Id: I9418dee7653ad7954c3788ecdd350fc8772edd32
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/301