Right now if we get an unexpected GOAWAY we print this in the exception hyper.http20.exceptions.ConnectionError: Encountered error 12, extra data b''.. This is fine, but we can do better.

We should bring in the error code registry from the spec so that we can print something more like this: hyper.http20.exceptions.ConnectionError: Error INADEQUATE_SECURITY (0xc): Negotiated TLS parameters not acceptable.

Ideally, the exception itself should be more structured, allowing easy access to numerical error codes, textual error names, long-form error descriptions, and the extra data.

I suspect the format string is roughly going to be "Error {s:name} ({d:errno}): {s:long_error}", where long_error is the extra data if it's available, and the description from the spec if it's not available.