Log info for missing values instead of showing errors.

[tmooney]

If an optional output had no value (i.e., was null in the JSON), then we'd see this error:

[ERROR] Don't know how to download type <class 'NoneType'>. Full object: None

Now instead this case will log an info message like:

[INFO] Skipping optional output that wasn't defined: some_optional_value

[chrisamiller]

+1 - nice improvement