[fields] Format error message only if params exist (encode#6624)

This prevents exceptions when the error message contains `%`, but is
not intended for formatting.  Django itself does the same:
https://github.com/django/django/blob/6866c91b638de5368c18713fa851bfe56253ea55/django/core/exceptions.py#L168-L169

Fixes encode#6622

rest_framework/fields.py

@@ -231,12 +231,12 @@ def get_error_detail(exc_info):
         error_dict = exc_info.error_dict
     except AttributeError:
         return [
-            ErrorDetail(error.message % (error.params or ()),
+            ErrorDetail((error.message % error.params) if error.params else error.message,
                         code=error.code if error.code else code)
             for error in exc_info.error_list]
     return {
         k: [
-            ErrorDetail(error.message % (error.params or ()),
+            ErrorDetail((error.message % error.params) if error.params else error.message,
                         code=error.code if error.code else code)
             for error in errors
         ] for k, errors in error_dict.items()

tests/test_fields.py

@@ -2255,14 +2255,33 @@ class ExampleSerializer(serializers.Serializer):
             password = serializers.CharField()
 
             def validate_password(self, obj):
-                err = DjangoValidationError('exc_msg', code='exc_code')
+                err = DjangoValidationError(
+                    'exc_msg %s', code='exc_code', params=('exc_param',),
+                )
                 if use_list:
                     err = DjangoValidationError([err])
                 raise err
 
         serializer = ExampleSerializer(data={'password': 123})
         serializer.is_valid()
-        assert serializer.errors == {'password': ['exc_msg']}
+        assert serializer.errors == {'password': ['exc_msg exc_param']}
+        assert serializer.errors['password'][0].code == 'exc_code'
+
+    @pytest.mark.parametrize('use_list', (False, True))
+    def test_validationerror_code_with_msg_including_percent(self, use_list):
+
+        class ExampleSerializer(serializers.Serializer):
+            password = serializers.CharField()
+
+            def validate_password(self, obj):
+                err = DjangoValidationError('exc_msg with %', code='exc_code')
+                if use_list:
+                    err = DjangoValidationError([err])
+                raise err
+
+        serializer = ExampleSerializer(data={'password': 123})
+        serializer.is_valid()
+        assert serializer.errors == {'password': ['exc_msg with %']}
         assert serializer.errors['password'][0].code == 'exc_code'
 
     @pytest.mark.parametrize('code', (None, 'exc_code',))