[Improvement] Check existence of BackupRepo before creating a Backup

[shanshanying]

Kubernetes: v1.27.3
KubeBlocks: 0.8.0-alpha.9
kbcli: 0.8.0-alpha.9

There is no backup repo by default.
When one creates a cluster, say mysql raft-group cluster, and creates a backup for that cluster.
He won't any error message until he checks the status of the newly created backup CR.

1. make sure there is no backup-repo

kbcli backuprepo list
No backup repository found

2. create a mysql cluster

kbcli cluster create mysql mycluster

3. create a backup

kbcli cluster backup mycluster --method xtrabackup

and this backup failed due to error: no default BackupRepo found

we'd better check the existence of backuprepo ahead.

[github-actions]

This issue has been marked as stale because it has been open for 30 days with no activity