Print message when Relaton fails to fetch document

[skalee]

If Relaton returns nil for given document query, that means one of following:

• Relaton does not support given type of document yet (feature request needed)
• There is a bug Relaton (bug report needed)
• It wasn't a document ref (source needs to be fixed or given pattern should be ignored)

In either case, debug message is potentially useful.

[skalee]

I need to re-think correctness of this pull request.