[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
Sure, you can have a look at the code. I just think that this isn't a nice experience from the user's (the developer that uses Symfony) point of view. I feel that the best experience is when one just reads the API documentation and you do know how to use a particular class and which implications its usage has.

Given this method, it's useful to know what the ``clearSession`` argument is used for, why I should set it to ``false`` and what it'll imply if I do that (so maybe my suggestion doesn't fit right how it should be done).

Though I'm not sure if this pull request is the right place to discuss this topic since it's somehow is related to the API documentation as a whole, isn't it?