Improve Error/Load State Handling

### Description

https://github.com/FlowFuse/node-red-dashboard/issues/443#issuecomment-2109003981

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