Repurpose Validate

[joepio]

The validate function used to be handy, as the store contained (unvalidated) string representations of atoms. Now, however, it only contains valid data, and Validate kind of lost its purpose. Perhaps I should remove it entirely, but It also had its use. It was able to detect multiple issues with some graph, and generate a report. Now, it throws an error as soon as it detects one.

[joepio]

I've changed error handling a couple of commits ago to improve the /validate endpoint. However, it now returns an error as soon as it encounters one, instead of telling how many errors there are. I'm not sure if these reports with multiple errors actually have that much benefit. I'm also considering dropping AD3 for another serialization format (#79).

[joepio]

#333 is related.