[Filesystem] introduced new Exception base classes

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.

The switch to the FilesystemInterface will allow you to have an implementation accessing S3 or Dropbox, etc. and still inject it into a classes, which are requiring the Filesystem.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| Doc PR        | symfony/symfony-docs#2947