fix #177 by actually calling the disconnect function instead of using always the fallback error-disconnect

(the if-condition was never met, since the command itself will never set the state to CA_DISCONNECTED)
SCR_StopCinema gets already called within Cl_Disconnect
the error disconnect now only gets used if the disconnect function doesn't suceed to set CA_DISCONNECTED
remove dead code - the disconnect function will never return without setting ca_disconnect, unless the client is not running, in which case we dont need to disconnect it anyway
make server shutdown on ERR_DROP again like before, as it is apparently expected in several locations within the server code
remove unused (and even abused) error type and its handling
print out server disconnection messages like results of a kick or ban to console as well
fix #178: cvar ui_stages leaks stage-information about both teams to everone
Merge remote-tracking branch 'tim/bug/#177-disconnect-cmd-error'
Merge remote-tracking branch 'tim/bug/#178-ui_stages'