Produce a better error when sending HEADERS on a quiesced connection

If a connection is quiesced remotely (by receiving a GOAWAY frame) and we attempt to open a new stream via a HEADERS frame, we receive a `NoSuchStream` error. This isn't a great error message: it's very hard to understand what has gone wrong. We should emit something more useful.

Trace the HEADERS handling path for a connection that has received GOAWAY, focusing on where the NoSuchStream error is produced. Define the work as replacing that result with an error that explains the connection is quiesced and verify the behavior with the relevant HTTP/2 tests.

Written by the indexing model from the issue text.