[4.2] Use FileNotFoundException

`get` or `getRequire` functions both check that the file exists and throw a `FileNotFoundException` if this not the case. So using `files->exists(...)` and thus checking twice that the file exists is useless.
Instead a `FileNotFoundException` can simply be caught.
Why?

PhpStorm for example automatically sorts them alphabetically every time new class is imported. Having need to manually resort them each time doesn't really help being productive.