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

Firewall/ContextListener.php

@@ -255,7 +255,7 @@ protected function refreshUser(TokenInterface $token)
             return null;
         }
 
-        throw new \RuntimeException(sprintf('There is no user provider for user "%s".', $userClass));
+        throw new \RuntimeException(sprintf('There is no user provider for user "%s". Shouldn\'t the "supportsClass()" method of your user provider return true for this classname?', $userClass));
     }
 
     private function safelyUnserialize($serializedToken)

RememberMe/AbstractRememberMeServices.php

@@ -233,7 +233,7 @@ final protected function getUserProvider($class)
             }
         }
 
-        throw new UnsupportedUserException(sprintf('There is no user provider that supports class "%s".', $class));
+        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?', $class));
     }
 
     /**