Is your feature request related to a problem?

I'm frustrated when nghttp2 malfunctions but sh2lib is silent.

Describe the solution you'd like.

Add some simple callbacks that print error codes/events and register them in do_http2_connect():

1. nghttp2_session_callbacks_set_error_callback2(callbacks, callback_error2),
2. nghttp2_session_callbacks_set_on_frame_not_send_callback(callbacks, callback_on_frame_not_send)
3. nghttp2_session_callbacks_set_on_invalid_header_callback2(callbacks, callback_on_invalid_header2)