Describe the bug
No error message is presented when a user takes a backup when incorrect backup target is entered

To Reproduce

• Steps to reproduce the behavior:
• Give. an incorrect value in Backup target in Settings page on Longhorn.
• create a volume, attach it to a workload, write data into the volume.
• Go to volume details page on Longhorn UI.
• Click on create. backup.
• Backup creation fails

Expected behavior
As the backup creation fails because of incorrect backup target data in the settings page, an error should be displayed.

Environment:

• Longhorn version: master
• Kubernetes version: 1.17.4
• Node OS type and version: rke DO