[Debug] Fixed ClassNotFoundFatalErrorHandler which could cause a fatal error

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9104
| License       | MIT
| Doc PR        | -


It might happen that `require_once` will include a file second time (for example with links or case-changed paths). Therefore we need to check if a class exists before requiring the file.

I also removed an unused argument from two methods (`$loader`).