React to errors on socket correctly when using epoll().

React to errors on socket correctly when using epoll().

Even though the socket is not registered for wxFDIO_EXCEPTION events,
wxEpollDispatcher will currently generate them anyhow so we shouldn't assert
if wxSocketImplUnix::OnExceptionWaiting() is called.

Instead, notify the socket about the connection loss if this happens.

Closes #11085.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61813 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775