fix(GrafanaFolder): proper status updates for removed CRs, don't fail on missing folder in onFolderDeleted events

[weisdd]

Two fixes for grafanafolder controller:

• Previously, onFolderDeleted would treat a missing folder (e.g. deleted through Grafana UI) as an error, which should have never been the case. - We just overlooked a missing ! in error handling logic;
• When cleaning up status for CRs that had been deleted, syncFolders was supposed to check if a folder still exists in Grafana, but instead of using method for folders, it used one for dashboards.

Two manual test cases to verify the fix:

• onFolderDeleted:
  • Steps:
    • deploy GrafanaFolder CR (e.g. from examples/folder);
    • Grafana CR should contain a reference to the folder in status.folders;
    • remove the folder from UI;
    • remove the folder CR;
  • Expected results:
    • No errors;
    • No event requeues;
    • Grafana CR should no longer contain a reference to the folder in status.folders;
• syncFolders:
  • Steps:
    • deploy GrafanaFolder CR (e.g. from examples/folder);
    • Grafana CR should contain a reference to the folder in status.folders;
    • scale down the operator (kubectl scale deployment grafana-operator-controller-manager-v5 --replicas 0);
    • remove the GrafanaFolder CR;
    • scale up the operator (kubectl scale deployment grafana-operator-controller-manager-v5 --replicas 1);
  • Expected results:
    • No errors;
    • Log message "folder no longer exists";
    • Grafana CR should no longer contain a reference to the folder in status.folders.

Fixes: #1512

[theSuess]

Thanks for catching this!