Use RecordNotFoundError exception when record is not found.

[nsheff]

In many places, I'm finding that retrieve_one will return None if the record_identifier is bad.

It would be more convenient for me if it would raise a RecordNotFoundError (which already exists), and I think is what this exception type was meant for. Then I can catch this and return a coherent message to the user. With just returning "none" silently, I'm having to build more error handling code.

I think there are several places that it just returns None -- maybe these should instead raise appropriate exceptions?

[donaldcampbelljr]

Fixed:
363a54c

[donaldcampbelljr]

Closed via #92