Allow gallery.py to serve original images.

Allow gallery.py to serve original images.

This reworks some of the internals to clean up URL validation and
error handling.  Rather than pass `stream` around, we raise the new
HTTPError wherever we want.  These errors are caught and dealt with
higher up, so low level internal functions don't need to care.  That
way I don't get nervous the next time I forget about
ProcessingComplete ;).