Merge pull request #1720 from dinhnv/fix_py3_exception_message

Fix #1715 python3 exception has no attribute 'message'

flask_admin/model/base.py

@@ -1555,7 +1555,7 @@ class BaseModelView(BaseView, ActionsMixin):
         try:
             self.on_model_change(form, model, is_created)
         except TypeError as e:
-            if re.match(r'on_model_change\(\) takes .* 3 .* arguments .* 4 .* given .*', e.message):
+            if re.match(r'on_model_change\(\) takes .* 3 .* arguments .* 4 .* given .*', str(e)):
                 msg = ('%s.on_model_change() now accepts third ' +
                        'parameter is_created. Please update your code') % self.model
                 warnings.warn(msg)