Improved error message when index in __getitem__() is invalid.

django/forms/boundfield.py

@@ -63,7 +63,10 @@ class BoundField:
         # Prevent unnecessary reevaluation when accessing BoundField's attrs
         # from templates.
         if not isinstance(idx, (int, slice)):
-            raise TypeError
+            raise TypeError(
+                'BoundField indices must be integers or slices, not %s.'
+                % type(idx).__name__
+            )
         return self.subwidgets[idx]
 
     @property