[Mailer] Fix string-cast of exceptions thrown by authenticator in EsmtpTransport

Author: wikando-ck

Transport/Smtp/EsmtpTransport.php

@@ -173,7 +173,7 @@ private function handleAuth(array $modes): void
                 }
 
                 // keep the error message, but tries the other authenticators
-                $errors[$authenticator->getAuthKeyword()] = $e;
+                $errors[$authenticator->getAuthKeyword()] = $e->getMessage();
             }
         }