http2: throw better error when accessing unbound socket proxy

Fixes: https://github.com/nodejs/node/issues/22268

PR-URL: https://github.com/nodejs/node/pull/22486


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>