[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
As i was creating it i talked to @weaverryan and he had some other ideas what the class might do in future. At the moment i cant recall all of the things he told me. On the other hand, if it expends it might be better to call it `AuthenticationUtils` as suggested.