"Cannot connect to integration manager" screen should have more precise instructions

### Your use case

The integration manager currently shows a pretty generic error should the request to `POST /register` fail in any way:


This gives users little understanding of what is actually wrong, but the API does actually tell us a good deal about what might be wrong.

#### What would you like to do?

Present a different error should the integration manager be actually down (connection timeout / 404 / DNS issues) v.s. an error response from the integration manager. The Integrator integration manager provided by Element uses a standard `error` / `errcode` response format for errors, so we could use that to trigger more useful information. For instance:

`M_UNKNOWN` / `Failed to discover homeserver` means that their is a federation issue, and the user should be instructed to deal with that (contact their administrator / hosting provider?)

#### Why would you like to do it?

There is a high volume of support tickets coming in from self hosters asking why the integration manager isn't working, and most of the times these issues can be solved by the administrator of the server. It's seldom something that Element can help with.

#### How would you like to achieve it?

Parse the error from the response of the API, and provide more useful instructions. I.e. try the federation tester.

### Have you considered any alternatives?

I have not.

### Additional context

_No response_