block: bdrv_insert_node(): fix and improve error handling

 - use ERRP_GUARD(): function calls error_prepend(), so it must use
   ERRP_GUARD(), otherwise error_prepend() would not be called when
   passed errp is error_fatal

 - drop error propagation, handle return code instead

 - for symmetry, do error_prepend() for the second failure

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210920115538.264372-3-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>