Return a 403 if a view class's is_accessible returns false.

is_accessible says: Override this method to add permission checks.
This means that it should return forbidden since the item exists but you
are not allowed to see it

README.rst

@@ -35,4 +35,4 @@ If you want to localize administrative interface, install `Flask-BabelEx <https:
 Examples
 --------
 
-The library comes with a quite a few examples, you can find them in the `examples <https://github.com/mrjoes/flask-admin/tree/master/examples` directory.
+The library comes with a quite a few examples, you can find them in the `examples <https://github.com/mrjoes/flask-admin/tree/master/examples>` directory.

flask_admin/base.py

@@ -297,7 +297,7 @@ class BaseView(with_metaclass(AdminViewMeta, BaseViewClass)):
                 View function arguments
         """
         if not self.is_accessible():
-            return abort(404)
+            return abort(403)
 
     @property
     def _debug(self):