handle parsing for error when chunked

[stevenou]

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So I based this off of #480 which seems like it might be abandoned at this point. I copy-pasted the spec minus a minor fix for splitting the json into 2 chunks (it was missing a character in the 2nd chunk).

The approach taken here assumes the error is parsable json. We accumulate the chunks until the message is parsable, then we raise an error. I don't really know enough about SSE/streaming to know whether this is definitely a bad idea but none of the existing specs broke so I'm hoping it's acceptable.

Not sure what would happen if, let's say, the stream disconnects midway so we never get the completed error message. I'm guessing in that case a different type of error (related to connection/streaming) will surface instead of the original error, which is probably ok?

Or if the error is never parsable - then possibly the whole thing fails silently?

Let me know your thoughts please.