R1:6b4887ab224a

Improve error and large file handling in thumbnailing

Summary:
Ref T2479, T4406. We should do a better job of (a) handling image processing errors and (b) declining to process large image files.

This fixes the worst of it, which is that users can upload huge GIFs with a large number of frames and hang a `convert` process for a long time, eating a CPU and a pile of memory.

This code is still pretty iffy and needs some more work. A near-term product goal for it is supporting 100x100 profile images.

Test…