Purge fails if unreferenced file to remove is already missing

Not sure if this is the expected behavior or not to just fail instead of continue, but it seems that there is possibly some missing exception catching around unlinking of files?

```shell
$ php bin/satis purge ../satis.json /path/to/repository
File '/path/to/repository/include/all$4ab2d531543982a6cac0c06a846341276ba2c450.json' does not exist, defined in "includes" in '/path/to/repository/packages.json'
In PurgeCommand.php line 131:

unlink(/path/to/repository/dist/org/package/org-package-3fddb6037bea4aee926
5210f76e8e64068eb076e-zip-a8e20d.zip): No such file or directory

purge [ [ []]]
```