"Http response at 400 or 500 level" error should set a proper error code

When the server is anavailable or otherwise the network is unreachable, grpcweb reports code=UNKNOWN, message="Http response at 400 or 500 level". It should report more proper error code. Most of the clients are better served by setting code=UNAVAILABLE in this case.

GRPC defines the following mapping:

https://grpc.github.io/grpc/core/md_doc_http-grpc-status-mapping.html

Perhaps grpcweb should implement this spec.