Make FileExistsError less cryptic

Usual
```
Traceback (most recent call last):
  File "./owncloud.py", line 64, in <module>
    main()
  File "./owncloud.py", line 60, in main
    scaffolder.scaffold(args, args.templates, getcwd())
  File "/home/kondou/git/owncloud_scaffolding/owncloud_scaffolding/scaffolders/appscaffolder.py", line 103, in scaffold
    os.mkdir(appFolder)
FileExistsError: [Errno 17] File exists: '/home/kondou/git/owncloud_scaffolding/test'
```
is not helpfull stuff.