Validate document count

It's currently possible to legalise a document by post where the document count is zero.
This adds validation to catch any calculations where the document count is present and less than 1.
Test zero document count transaction
Test that a document count of zero throws the approriate error
Add invalid registration count exception
Remove document_count check

The document count is currently always present in transactions,
there's no need to test for this key.
Validate registration count where approriate
Merge remote-tracking branch 'origin/validate-document-count'