Fixed #36618 -- Corrected error message in BaseForm.add_error().

The error message now correctly states that the error argument
is a dictionary.

Author: nileshpahari

django/forms/forms.py

@@ -288,7 +288,7 @@ def add_error(self, field, error):
             if field is not None:
                 raise TypeError(
                     "The argument `field` must be `None` when the `error` "
-                    "argument contains errors for multiple fields."
+                    "argument is a dictionary."
                 )
             else:
                 error = error.error_dict

tests/forms_tests/tests/test_forms.py

@@ -1660,6 +1660,13 @@ def clean(self):
         with self.assertRaisesMessage(ValueError, "has no field named"):
             f.add_error("missing_field", "Some error.")
 
+        with self.assertRaisesMessage(
+            TypeError,
+            "The argument `field` must be `None` when the `error` argument is a "
+            "dictionary.",
+        ):
+            f.add_error("password1", ValidationError({"password1": "Some error."}))
+
     def test_update_error_dict(self):
         class CodeForm(Form):
             code = CharField(max_length=10)