b86f2bb22c errors if expecing headers notification but not subscribed (Riccardo Casatta)
Pull request description:
the opposite, erroring when subscribing multiple times, is not handled because clients could call multiple times to trigger the server to reply with the tip
ACKs for top commit:
danielabrozzoni:
utACK b86f2bb22c - changes look good to me, didn't test locally
Tree-SHA512: e1a21223448e708cc054271eb1ac5285dc98bdadf7497a856fc4a19ff51655879352ca6ef9f3fb5ec5f9071dd7ee50b2e44d4c65e70694e5d9fa53d280179dd2