Refined underlying error handling.

Exising error handler is for listen and the first accept.
After error handler is called, the server no longer accept.
If user needs to re-accept, call listen().
There is no change from the original behavior.

The new callback is added that takes ioc_context reference parameter.
It can help io_context management when user uses ioc_con_getter.

The error handler stop accept. But TLS/WS has aditionall errors.
For example, timeout after TCP connection has been established.
In this case, just accepted connection is error.
And then continue new accepting sequence.
User shouldn't call listen() in this case.

In order to disstinguish this error, connect_error_handler is added.

Author: redboltz