added information on AuthenticationFailureHandlerInterface

The Api Keys documentation made no mention of the AuthenticationFailureHandlerInterface which is required to correctly display Authentication Failure responses. Without it, authentication failures will result in a 500 response. I've made mention to the interface and given an example implementation.

http://symfony.com/doc/current/cookbook/security/api_key_authentication.html#cookbook-security-api-key-config

```
Doc fix? yes
New docs? no
Applies to: 2.4
Fixed tickets: none found
```