api: Make a structured exception type for API errors, and use it

[gnprice]

Like the ApiError and its base class RequestError that we have in the RN app in src/api/apiErrors.js.

[chrisbobbe]

Some API errors are for conditions that we'll want to recognize as fatal to the current API connection: the user's auth credential was invalidated, the realm or user was deactivated (see zulip/zulip-mobile#4571), possibly more. (Maybe the invalid-event-queue error?)

When working on this issue, I wonder if we need to think about those API errors differently than more mundane ones, like when you tried to send a message with a too-long topic.