[Filesystem] Now based on Interface and new Exception

The Filesystem class now throws a ```FileNotFoundException``` if a file could not be found, rather than an basic ```IOException```. The new exception is still a child of the ```IOException```, this way it doesn' t breack BC.
The ```IOException``` now also takes as the first argument an path to the file of interest, which can be used via the ```getPath()``` method.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no (only the IOException has been changed a bit)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| Doc PR        | symfony/symfony-docs#~~