Add IO tests for invalid inputs

[sciwhiz12]

Our current battery of tests check that the various IO modules serialize and deserialize the data without any unwanted change or errors.

However, for wider coverage, we should also add tests for checking that invalid inputs -- missing required fields, out-of-range indexes -- are caught on deserialization.

I'm not sure how those same tests would look for serialization, if they're needed at all for it (as ideally, our data-holdings objects should be validating their arguments are as expected).