Improve exception message for str.format (GH-12675)

Objects/stringlib/unicode_format.h

@@ -440,8 +440,13 @@ get_field_object(SubString *input, PyObject *args, PyObject *kwargs,
 
         /* look up in args */
         obj = PySequence_GetItem(args, index);
-        if (obj == NULL)
-            goto error;
+        if (obj == NULL) {
+            PyErr_Format(PyExc_IndexError,
+                         "Replacement index %zd out of range for positional "
+                         "args tuple",
+                         index);
+             goto error;
+        }
     }
 
     /* iterate over the rest of the field_name */