Add tests to ensure the key path of errors gets returned.
Fix a typo in the expected path of the error.
Add method to JSONModelError that creates a duplicate error object with a key-path-component in the userInfo dictionary.
Propagate the key-path of errors in the user info dictionary.
Clarify test-failure message.
Add error-path checks to a few other test-cases.
Merge pull request #104 from j-h-a/nested-error-paths

Propagate the key-path of any errors out to the end-user in the user info dictionary.