Ignore ontologies if they repeatedly don't validate

[caufieldjh]

As noted in #23 (comment), some ontologies fail validation at merge time consistently due to unresolved structural issues. They should be added to an ignore list so we don't repeatedly attempt to merge them.

Some fail because they have empty edgefiles. This sounds like a problem for an ontology, but that's only because some of these are more like controlled vocabularies. We may try to merge them anyway.