Raise exceptions for `{"result": "error"}` in `zulip` apis.

[LoopThrough-i-j]

Currently, zulip API doesn't handle {"result": "error"} and sends send back whatever comes from the server. We would want to handle these errors by raising exceptions in the API layer whenever there is a {"result": "error"}.

Reference

[alexzw7]

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[alexzw7]

@zulipbot add "good first issue"

[alexzw7]

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[LoopThrough-i-j]

@zulipbot remove "good first issue"

[PIG208]

A brief note for this one here: we might need to add a common exception class called BotError and specify a set of sub errors like AccessDenied etc. (we need to discuss the naming). I think we can have a protocol that can be shared with embedded bots as well.
An anticipated result of this might be that the user will no longer need to examine the returned dictionary from the API and instead they can simply do error handling with try-except statements.

[timabbott]

Yeah, I think that's a good idea.

[neiljp]

Is it the intent of this issue to evolve all of the zulip python API to have different return values and raise exceptions instead?
While ZT can adapt to this per zulip library release, this seems like a big API transition to make for all dependent packages/scripts.

If this is an evolution of zulip, then BotError seems misnamed, unless this issue is specific to the bots library instead?