migration/rdma: Don't flag an error when we've been told about one

If the other side tells us there's been an error and we fail
the migration, we don't need to signal that failure to the other
side because it already knew.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael R. Hines <michael@hinespot.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>