COMMON: Modify Base::processSettings, get rid of Common::kArgumentNotProcessed

Instead of defining a hacked up Common::Error code, split the return
value of processSettings into two parts: An error code, and a value
which indicates whether the specified command was completely handled
by processSettings or not.

common/error.cpp

@@ -67,9 +67,6 @@ static String errorToString(ErrorCode errorCode) {
 	case kEnginePluginNotSupportSaves:
 		return _s("Engine plugin does not support save states");
 
-	case kArgumentNotProcessed:
-		return _s("Command line argument not processed");
-
 	case kUnknownError:
 	default:
 		return _s("Unknown error");