Avoid cryptic messages when there are no arguments

```
kondou:owncloud_scaffolding/ $ ./owncloud.py
Traceback (most recent call last):
  File "./owncloud.py", line 57, in <module>
    main()
  File "./owncloud.py", line 52, in main
    if scaffolder.canHandle(args):
  File "/home/kondou/git/owncloud_scaffolding/owncloud_scaffolding/scaffolders/scaffolder.py", line 52, in canHandle
    if args.which == self._cmd:
AttributeError: 'Namespace' object has no attribute 'which'
```
is not helpfull