Gracefully handle when the REST API is down

[hackaugusto]

Problem Definition

If the webui is loaded in the browser but REST API server is not running the interface is navigable but all listings are empty and the error "Http failure response for (unknown url): 0 Unknown Error" is shown.

TODO

Identify when the server is unreachable and add an overlay to the UI, which makes doesn't allow the UI to be used and informs the user.

[LefterisJP]

@kelsos Does this make sense to do in both the old UI and the new one or only in the new one?

[kelsos]

Actually I was thinking about this in the morning.
An "offline" screen with some central message that will inform the user either that the rest API is not reachable, or that RPC is not reachable.

I would say for the new UI, otherwise it will be duplicate effort. since a lot of the stuff has to change.