evio: make on_accept be nothrow

Evio is going to be C, because it is needed in SWIM to
1) support UNIX sockets in future;
2) do not care about setting SocketError directly. It
is not possible via bare sio, because sio_bind ignores
EADDRINUSE.

A first step to make evio C - eliminate exceptions
from its callbacks available to other modules. The
only callback it has - on_accept.

Needed for #3234