Description of problem:
rbd mirror snapshot schedule rm returns 0 when tried to remove non-existent schedule

this decreases usability, if a user makes any typo in his command, he wouldn't get to know that there has been some mishap and he wouldn't be achieving what exactly he wished to achieve.

Version-Release number of selected component (if applicable):
16.2.0-117.el8cp

How reproducible:
Always

Steps to Reproduce:
1. Create two clusters with rbd snapshot based mirroring enabled
2. Try to delete non-existent schedule

Actual results:
[ceph: root@magna008 /]# rbd mirror snapshot schedule ls -p two_way --recursive
POOL     NAMESPACE  IMAGE  SCHEDULE          
two_way  -          -      every 1h, every 6h
[ceph: root@magna008 /]# rbd mirror snapshot schedule rm  --pool two_way --image image 7h
[ceph: root@magna008 /]# echo $?
0


Expected results:
Error message should be displayed if user tries to delete non existent schedule

Additional info: