[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        | ---
this is wrong. It does not return a string but an ``AuthenticationException`` (or ``null``)