Make mutt_copy_message distinguish between fatal and non-fatal errors.

Make mutt_copy_message distinguish between fatal and non-fatal errors.
Non-fatal errors should prevent moving messages, since they indicate
data loss. But mutt should still attempt to display them, since being
able to see some attachments is better than nothing.
Also stop printing out non-PGP material in application/pgp
attachments. Closes #2545, #2912.