Console: Improved output after invalid use of command arguments

The message can be more specific as to what was the problem.

doomsday/client/src/con_main.cpp

@@ -1082,7 +1082,15 @@ static int executeSubCmd(const char *subCmd, byte src, boolean isNetCmd)
     }
 
     // What *is* that?
-    Con_Printf("%s: unknown identifier, or command arguments invalid.\n", args.argv[0]);
+    if(Con_FindCommand(args.argv[0]))
+    {
+        LOG_WARNING("%s: command arguments invalid") << args.argv[0];
+        Con_Executef(CMDS_DDAY, false, "help %s", args.argv[0]);
+    }
+    else
+    {
+        LOG_MSG("%s: unknown identifier") << args.argv[0];
+    }
     return false;
 }