Improve some error messages for functions of exactly one variable

This changes
  "the value of the first argument"
to
  "the value of the only argument"
in some type-error error messages for functions accepting exactly
one argument.

src/c/string.d

@@ -241,7 +241,7 @@ cl_string(cl_object x)
       break;
     }
   default:
-    FEwrong_type_nth_arg(@[string],1,x,@[string]);
+    FEwrong_type_only_arg(@[string],x,@[string]);
   }
   @(return x);
 }