On ExceptionController getExceptionMessage() fallback to error

On ExceptionController getExceptionMessage() fallback to 'error' if the status code is not recognized.

On getParameters() code I notice this:
```php
            'status_text' => array_key_exists($code, Response::$statusTexts) ? Response::$statusTexts[$code] : "error",
```

I think is good idea to do the same on getExceptionMessage(), this PR is related with the code introduced on
https://github.com/FriendsOfSymfony/FOSRestBundle/pull/647