Fix ErrorWriter to be codec agnostic

[emcfarlane]

This PR changes the ErrorWriter to be more lenient with classifying protocols. Errors codecs are agnostic to the codec used. Therefore we avoid checking the codec in classifying the request. IsSupported will return true for an unknown codec which allows the server to encode a better error message to the client. If not supported a 415 error response could be used to match gRPC server like handling. If not supported and trying to write an error the ErrorWriter will default to connects unary encoding (consistent with current behaviour).

Fixes #689