raw-h2: handle server closing connection

[Dima Tisnek]

Today

Bad error <ConnectionTerminated error_code:ErrorCodes.NO_ERROR, last_stream_id:0, additional_data:7b22726561736f6e223a22426164436572746966>

The hex translates to {"reason":"BadCertif...

To do

• report connection being closed
• decode reason in additional data
• [possibly] treat this as graceful shutdown, when h2 is ready python-hyper/h2#1181
• [possibly] refuse to open new connections with same cert, or
• [possibly] throttle connection open rate, as not to flood the server

[Dima Tisnek]

done, except graceful shutdown.