Should we throw a better exception instead of logging, eating and returning a null object in GCToolkit main class

[karianna]

          Should we throw a better exception instead of logging, eating and returning a null object?

Originally posted by @karianna in #252 (comment)

[dsgrieve]

There should be API for consuming errors from the parser and from the aggregators/aggregations. As a user of the toolkit, I should be able to receive some notification of an error or exception. There is (or will be soon) API for the event and parsing message bus. There should (or could) be an error message bus. By default, the consumer could just log. Or I could implement a consumer that take some action when an error is posted.