Merge branch '3.4' into 4.3

* 3.4:
  [Validator] fix access to uninitialized property when getting value
  [HttpKernel] Fix stale-if-error behavior, add tests
  Improved error message when no supported user provider is found

Author: nicolas-grekas

User/ChainUserProvider.php

@@ -91,7 +91,7 @@ public function refreshUser(UserInterface $user)
             $e->setUsername($user->getUsername());
             throw $e;
         } else {
-            throw new UnsupportedUserException(sprintf('The account "%s" is not supported.', \get_class($user)));
+            throw new UnsupportedUserException(sprintf('There is no user provider for user "%s". Shouldn\'t the "supportsClass()" method of your user provider return true for this classname?', \get_class($user)));
         }
     }