fix UnboundLocalError in handle_url_build_error

- caused by changes in the execution model of python 3 where the alias of an except clause is cleared on exit of the except

flask/app.py

@@ -1631,8 +1631,9 @@ class Flask(_PackageBoundObject):
                 rv = handler(error, endpoint, values)
                 if rv is not None:
                     return rv
-            except BuildError as error:
+            except BuildError as e:
-                pass
+                # make error available outside except block (py3)
+                error = e
 
         # At this point we want to reraise the exception.  If the error is
         # still the same one we can reraise it with the original traceback,

flask/testsuite/basic.py

@@ -772,6 +772,17 @@ class BasicFunctionalityTestCase(FlaskTestCase):
         with app.test_request_context():
             self.assert_equal(flask.url_for('spam'), '/test_handler/')
 
+    def test_build_error_handler_reraise(self):
+        app = flask.Flask(__name__)
+
+        # Test a custom handler which reraises the BuildError
+        def handler_raises_build_error(error, endpoint, values):
+            raise error
+        app.url_build_error_handlers.append(handler_raises_build_error)
+
+        with app.test_request_context():
+            self.assertRaises(BuildError, flask.url_for, 'not.existing')
+
     def test_custom_converters(self):
         from werkzeug.routing import BaseConverter
         class ListConverter(BaseConverter):