Changed to use more appropriate http status code for ResourceExhausted

[eleniums]

HTTPStatusFromCode should be mapping codes.ResourceExhausted to http.StatusRequestEntityTooLarge (https://httpstatuses.com/413) as it is the more appropriate HTTP status. The current mapping of http.StatusServiceUnavailable seems to indicate a temporary problem with the server, when in reality it is an issue with the client sending a request that is too large.

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[googlebot]

CLAs look good, thanks!

[johanbrandhorst]

Arguably the correct mapping is that defined by the Google protobuf error code proto file; 429 Too Many Requests.

[achew22]

+1, it should be 429. Could you also add a link to https://github.com/googleapis/googleapis/blob/master/google/rpc/code.proto in the function comment?

[johanbrandhorst]

Looks like there are more codes that should be chaged, FailedPrecondition should be 400 Bad Request, not 412 PreconditionFailed. This is probably a good time to review all the codes.

[achew22]

@johanbrandhorst, If you want to send in a PR that fixes it and updates the comment on the function I'm happy to merge it today.

[eleniums]

My initial reason for bringing this up was that the instances in grpc-go where ResourceExhausted is currently being used are related to message size, not the resource actually being exhausted. I thought that 413 PAYLOAD TOO LARGE would be a more appropriate mapping, given the current implementation.

That being said, in order to be consistent, it does make sense to map the codes based on https://github.com/googleapis/googleapis/blob/master/google/rpc/code.proto. I'll make the necessary adjustments and push a commit soon.

[achew22]

When Travis is done I'll merge. Please ping if I forget

[achew22]

Thanks so much @eleniums for contributing!