Add HTTP error code, error status to responseStreamChunk Error

[mwitkow]

When using unitary-to-streaming requests, it is hard to reason about HTTP error codes based on a chunk that contains error: it only contains the text of the error response.

Instead, it would be useful for the error field of a chunk to be structured:

• grpc_code containing the code of the RPC error (to avoid the pain of extracting it from the error string)
• http_code that is from HTTPStatusFromCode
• message containing the current string of the error
• http_status containing a textual representation of the grpc_code or http_code.

[mwitkow]

Yay, thanks @kdima!