migrate_incoming: Cleanup/clarify error messages

migrate_incoming: Cleanup/clarify error messages

Create a separate error for the case where migrate_incoming is
used after a succesful migrate_incoming.

Reword the error in the case where '-incoming defer' is missing
to omit the command name so it's right for both hmp and qmp.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>