[BUG] PDU widget stops displaying properly if room is deleted

[lunkwill42]

Describe the bug
A PDU widget that shows PDUs in a specific room will only display a Cannot load widget error if the room is deleted, which is not a very helpful hint about what has gone wrong.

To Reproduce
Steps to reproduce the behavior:

1. Go your dashboard and add a PDU widget.
2. Assuming you have PDUs in your NAV database, configure the widget to display the room with this/these PDUs in it.
3. Delete that room in SeedDB
4. Go back to dashboard
5. See error

Expected behavior
A more helpful error message, explaining that the configured room has disappeared, and that a new one should be configured.

Environment (please complete the following information):

• NAV version installed: 5.0.5