[SecurityBundle] error helper added symfony/symfony#11147

Added helper that extracts last authentication error and username.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [ #11147 ]
| License       | MIT
| Doc PR        | symfony/symfony-docs#3996
I would write something like this:

```php
/**
 * Returns the last authentication error.
 *
 * Authentication errors are read from the current request or fom the session. Keeping
 * errors in the session may cause issues when calling the method several times cause
 * you don't know if the errors has been read before. Therefore, after retrieving the error,
 * it is erased by default. If you want to keep the error in the session, you'll have to pass
 * false to this method.
 */
```

I know that the method name is quite self-explanatory, but when I look at the generated API documentation I find it useful if there is some more explanation.