bug #1266 Fix error never removed from session in SecurotyProvider (Savageman)

This PR was submitted for the master branch but it was merged into the 1.3 branch instead (closes #1266).

Discussion
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Fix error never removed from session in SecurotyProvider

When $message is different than the $error variable, the wrong session key is deleted.

Commits
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2509b5ac Fix error never removed from session in SecurotyProvider

src/Silex/Provider/SecurityServiceProvider.php

@@ -357,10 +357,10 @@ class SecurityServiceProvider implements ServiceProviderInterface
 
             $session = $request->getSession();
             if ($session && $session->has($error)) {
-                $error = $session->get($error)->getMessage();
+                $message = $session->get($error)->getMessage();
                 $session->remove($error);
 
-                return $error;
+                return $message;
             }
         });