Empty collections raise an unhelpful exception

[bitprophet]

The list/help mechanisms do not cope well with empty collection modules (eg touch whatever.py; inv -c whatever -l). Fix this.

Also related, I don't think we used to raise raw CollectionNotFound when tasks files could not be found, make sure that's not also broken.

[bitprophet]

I see nothing about that 2nd bit so it's probably its own issue. See #149.