client_migrate_info reports spice_server_migrate_connect() failure as
"An undefined error has occurred".  Improve to "Could not set up
display for migration".

QERR_UNDEFINED_ERROR is now unused.  Drop.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201113082626.2725812-6-armbru@redhat.com>