Thanks @JoelQ, good spot! I've added a step in my feature to check for the absence of "Errors:" in the output. I'm always at a bit of a struggle when writing cucumber acceptance tests...it's starting to feel a little imperative like *When I click here and Then I do this and Then I do this Then I should see this* you know...? :|

Maybe I should rewrite them to something like:
```gherkin
Given a puzzle "valid_complete.sudoku"
When I validate it
Then there should be no errors
```

and then those steps can call `step "When I successfully run..."`. But then I kinda like how the aruba steps document the command line interface...it's hard to balance!