Show detailed register-queue feedback

[gnprice]

This is like #555 but for the step where we call registerQueue, rather than getEvents.

On the implementation, some details in chat thread: https://chat.zulip.org/#narrow/stream/48-mobile/topic/0.2E0.2E19.20Flutter.20.3A.20Cant.20connect.20to.20self.20hosted.20instance/near/1923246

This will let us distinguish between:

• there's some kind of network issue (including a TLS-aka-SSL incompatibility, which can be subtle to diagnose)
• the data from the server is malformed — so either a bug in the client or in the server, and either way something we need to diagnose
• some other issue

and will give the user details they can send us in a screenshot. Particularly in the case of malformed server data, we'll want the stack trace so we can debug.

[gnprice]

A new issue today is a reminder of the malformed-server-data case:

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