Better error message if Author, Owner not in proper format

[appukuttan-shailesh]

If string input for author/owner, e.g.
author="Shailesh Appukuttan"
we get an error message of the form:

Exception Type: TypeError at /models/
Exception Value: string indices must be integers

This should be updated to provide a more informative response, such as:
author and owner fields should have values of the form:
{"family_name": "Appukuttan", "given_name": "Shailesh"}