ENG-6762 Improve no snapshot for restore log msg.

Provide explanatory text ahead of the specific error listing.
At Ruth's suggestion, include support@voltdb.com as contact.
Additionally, a knowledge base article has been proposed
with details for the specific error reasons that can be
listed as part of this log message.
Merge pull request #1625 from VoltDB/eng-6762

ENG-6762 Improve no snapshot for restore log msg.