Improve Error/Load State Handling

[joepavitt]

Description

FlowFuse/node-red-dashboard#443 (comment)

If a user tries to inject an invalid URL/site into the iframe, i.e. one that forbids iframe injection, then there is no user feedback. We should add a handler tot he iframe's @load event and handle accordingly. Ideally communicating to the user that the iframe has not yet loaded any content, or, if failed, why the content failed to load.

Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate