find_or_404 should only catch ValueError.

By catching all errors it makes it very hard to debug.  For example, if
the Redis service craps out in the middle of the request, a 404 will be
returned instead of a 500, which means the exception will be silently
ignored, and not being logged correctly.
Merge pull request #87 from dhrrgn/master

find_or_404 should only catch ValueError.