grpc-js: Improve reporting of HTTP error codes

[murgatroid99]

This makes a couple of changes to HTTP status code handling:

1. When surfacing a gRPC error synthesized from an HTTP status code, the details string now contains the actual status code number.
2. When a call ends with no trailers received and there was a non-OK HTTP status code, surface that error instead of a more generic error about not receiving trailers. This fixes grpc-js doesn't handle HTTP/2 error responses with headers and a body and no trailers #2722.

I also added some information to the "RST_STREAM with code 0" error to explain what it means semantically, and I had to refactor handleTrailers. That refactor does change the behavior slightly, mainly to prioritize the gRPC status over the HTTP status when provided, which I think is more correct behavior.