919134 - Add explaination to BadRequest raised when distributor not inst...

https://bugzilla.redhat.com/show_bug.cgi?id=919134

This fix is fine for now but we really need a more comprehensive approach to validating that the child node is suitable for synchronization.  I incorporated this work on a branch (jortel-nodes-reporting) making improvements to node update summary reporting in general.  But, the changes on that branch became to widespread and could not be completed and reviewed this sprint.