Structured output + more informative error messages

[wpears]

To go along with #84, the loader needs a more structured output than can be harvested for script-level decisions (retry only x, y, and z files from a directory of many).

Also, should resurrect the logging-on-skipped data feature, but maybe only report the skipped data at the end of loading (so just buffer it and log the buffer as part of the featurecount !== expected message

The goal is to never wonder where something went wrong in the loading/retrieving process, but always be informed via structured output that can be acted on by humans or machines.

[wpears]

Will also help solve #104