TLS should log errors

[Sec42]

Is your feature request related to a problem? Please describe.
When using the tls+tcp:// protocol I can not tell whether the connection does not work because the other side is not listening, or if the tls handshake is not working.

nng_pipe_notify() does not seem to have triggers for this case.

Describe the solution you'd like
Some way to get notified of the failed connection attempts. Preferably including a reason why the connection failed

Describe alternatives you've considered
It would probably be possible to do a connection attempt outside of nng to test for this case, but that seems convoluted.

Additional context
It would help me make my program more user-friendly to notify the end user of such a problem instead of waiting indefinitely for a connection.

[gdamore]

the problem is that nni_pipe_notify() is only able to pick up post-negotiation. I need to think about adding either logging hooks or some kind of other callback mechanism for TLS.

[gdamore]

If you're client side, nng_dial() should return an error, and the error code should discriminate between TLS and non-TLS errors (e.g. ECONNREFUSED). Getting more detail about the TLS error is harder. I'll think on this.