[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'm against documenting WHAT the code is doing. It's redundant, since you can read what the code is doing from the code itself... Also, this kind of comments often lie, since people tend to forget to update them. 

We should rather explain WHY we're doing something, or document potential pitfalls. Anything that's not self-evident from reading the code.