CLI tests

[nkrabben]

This is for #161

I ended up noticing a couple of bugs and unclear behaviors, mostly around completeness tests. I changed the requirement for --processes to 1 or more since setting it to 0 make multiproccesing throw errors. I also changed the error message for failed completeness validations to Bag is incomplete to differentiate it from failed hash validations.

The tests are a bit verbose and use nested with's to capture the different outputs. If there is a cleaner way, I'm happy to use that instead.

[nkrabben]

Force push because I accidentally committed my test file in a9b769d