Added unit tests for unzip and added a user-friendly error message for unzip being unable to perform
its duties of extracting a project.json from the given input (zip).
Refactor unpack and main exported function to return optional zip, if originally provided, in
addition to the validated project.
BREAKING CHANGE: Change to main api to return originally provided zip (or null if string was
provided) along with validated project, in a 2-element array.