task/workunit: clean up dir deletion

task/workunit: clean up dir deletion

This was always trying to remove the mountpoint,
and then swallowing the exception and printing
it to the log.

Instead:
 * Ensure it's only trying to delete mnt if it created it
 * Don't swallow those exceptions: if deleting the dirs
   fails then something has gone wrong and it should bubble
   up.

Signed-off-by: John Spray <john.spray@redhat.com>