Improve target loader error messaging

[wekyle]

If I forget to add a required key to a target, I get this:

Oops! There was a problem parsing targets/example.js: 
'AssertionError: expected { Object (verbs, description, ...) } to have keys 'verbs', 'name', 'description', and 'workingDir''

While this is helpful, it would be nice to be more user-friendly.