Prevent HTTP 500 errors when failing to generate a thumbnail.

If a file for a file attachment is not found when generating a
thumbnail, an exception is raised that bubbles up, resulting in a HTTP
500 error. Now, we catch that error as part of the other operations we
were sandboxing, log it, and return an empty string instead of bubbling
the error up.

Reviewed at https://reviews.reviewboard.org/r/6434/