TODO: Improve connection close reporting

[jasnell]

When a QuicSession is closed with a received CONNECTION_CLOSE, make sure the connection close error code is transmitted out to the JavaScript side so the user can know why the connection was closed.

[jasnell]

There appears to be a limitation currently in the ngtcp2 where we cannot directly access the error_code from a received connection close... so while we can know that a connection has closed, we can't know exactly why yet. I've opened ngtcp2/ngtcp2#147 to get clarity.

[jasnell]

Addressed in #207