Switch an error msg away from using "i."

Although the new text is longer, I think it's more understandable
in English, in general, than the more German notation "i.".

src/gap.c

@@ -590,8 +590,8 @@ static Obj FuncWindowCmd(Obj self, Obj args)
     {
       tmp = ELM_LIST( args, i );
       if (!IS_INTOBJ(tmp) && !IsStringConv(tmp)) {
-          ErrorMayQuit("WindowCmd: %d. argument must be a string or integer "
-                       "(not a %s)",
+          ErrorMayQuit("WindowCmd: the argument in position %d must be a "
+                       "string or integer (not a %s)",
                        i, (Int)TNAM_OBJ(tmp));
           SET_ELM_PLIST(args, i, tmp);
       }

tst/testinstall/kernel/gap.tst

@@ -79,8 +79,8 @@ Error, WindowCmd: <cmd> must be a string (not the value 'fail')
 gap> WindowCmd([""]);
 Error, WindowCmd: <cmd> must be a string of length 3
 gap> WindowCmd(["abc",fail]);
-Error, WindowCmd: 2. argument must be a string or integer (not a boolean or fa\
-il)
+Error, WindowCmd: the argument in position 2 must be a string or integer (not \
+a boolean or fail)
 gap> WindowCmd(["abc"]);
 Error, window system: No Window Handler Present
 gap> WindowCmd(["abc",1,"foo"]);