mgr/dashboard: persist error task notifications to notifications page

Task notifications (isFinishedTask=true) were only shown as toasts
but never saved, making "View more" link navigate to an empty page.
Only error task notifications are now persisted to the notifications
page; success tasks remain toast-only.

Signed-off-by: Afreen Misbah <afreen@ibm.com>
Fixes: https://tracker.ceph.com/issues/79662