[imp] vfs "false positive" / imp attachments

[Liam Hoekenga]

We've run into a problem a couple of times that if the horde_vfs table gets
somehow corrupted (or is unavailable?), and someone attaches a file, IMP
erroneously reports "Attachment added".  I think this condition should probably
throw an error message..

I didn't try adding / updating the table (other than dropping it and recreating
it) in the mysql command line tool to see how it responded, but "select" and
"delete" took ages... (actually, after a couple of minutes of no response
during those queries, I cancelled them). 

Liam