Make modular transforms handle exceptions gracefully

Summary:
Ref T7707. Ref T2479. Ref T5258.

The thumbnailing code is some of the only code in the codebase which doesn't use exceptions to handle errors. I'm going to convert it to use exceptions; make sure they do something reasonable at top level.

Strategy here is:

• By default, we just fall back to a placeholder image if anything goes wrong.
• Later, I'll likely add a "debug" workflow from the new "Transforms" UI which will surface the specific exception instead (the code can't really raise any interesting exceptions right now).

Test Plan: Faked an exception and saw some reasonable default images.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5258, T2479, T7707

Differential Revision: https://secure.phabricator.com/D12809