🐛 Fixed error handling when running multithreaded

[AnotherFoxGuy]

Patch applied from RigsOfRods/ror-server@b455cec

Removed variable SWBaseSocket::error_string which was written on any error and read on error in disconnect(), all without synchronization. This is a problem in Rigs of Rods server which uses each client socket from 2 threads: receiver and broadcaster.
Function SWBaseSocket::set_error() was modified to only fill the supplied SWBaseError object, without touching anything in the SWBaseSocket object. Also, the function won't shut down the whole program with exit() anymore.
"verbose mode" was removed completely.

[tritonas00]

Fixes RigsOfRods/rigs-of-rods#1279