migration: Unify failure check for migrate_add_blocker()

migration: Unify failure check for migrate_add_blocker()

Most callers check the return value.  Some check whether it set an
error.  Functionally equivalent, but the former tends to be easier on
the eyes, so do that everywhere.

Prior art: commit c6ecec43b2 "qemu-option: Check return value instead
of @err where convenient".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-10-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>