Ignore AttachmentDataNotFound exception
Ignore AttachmentDataNotFound exception

This exception was introduced because sometimes the AttachmentData
doesn't exist by the time the asset is uploaded, so we need to try
again after a little while.

It's a suboptimal solution, but it seems to work.  Ideally there would
be an ordering constraint between creating the AttachmentData and
running the worker, but I'm not sure that's feasible right now.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall