Change: not performing a command is worthy of an error, not a warning

If a command cannot be executed for whatever reason, it makes no sense to call it a warning. Something has been done wrong.
Also make writing of these error message consistent while changing their "type".

src/framerate_gui.cpp

@@ -1076,6 +1076,6 @@ void ConPrintFramerate()
 	}
 
 	if (!printed_anything) {
-		IConsoleWarning("No performance measurements have been taken yet");
+		IConsolePrint(CC_ERROR, "No performance measurements have been taken yet.");
 	}
 }