Client does not report correct error codes per the spec for HTTP status code mapping

[jhump]

A well-formed gRPC response always has an HTTP status code of “200 OK”. The gRPC docs specify a table for mapping other HTTP status codes to gRPC error codes: https://github.com/grpc/grpc/blob/master/doc/http-grpc-status-mapping.md.

The client in this package varies from the above table. Specifically, here are HTTP status codes which produce an unexpected error code:

HTTP status code	Expected error code	Actual error code
400 Bad Request	internal	unknown
404 Not Found	unimplemented	unknown
409 Conflict	unknown	aborted
412 Precondition Failed	unknown	failed_precondition
429 Too Many Requests	unavailable	unknown
502 Bad Gateway	unavailable	unknown
504 Gateway Timeout	unavailable	unknown

[sampajano]

@jhump Thanks a lot for reporting! Will take a look soon!