migration: Display error in query-migrate irrelevant of status

Display it as long as being set, irrelevant of FAILED status.  E.g., it may
also be applicable to PAUSED stage of postcopy, to provide hint on what has
gone wrong.

The error_mutex seems to be overlooked when referencing the error, add it
to be very safe.

This will change QAPI behavior by showing up error message outside !FAILED
status, but it's intended and doesn't expect to break anyone.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2018404


Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231004220240.167175-2-peterx@redhat.com>