Log the reason why a ref object wasn't loaded by RevisionReader

It may happen that one ref cannot be read in memory for being used
in an ApplyObject REST-API. Add more logging on the reason behind
the failure to read all the objects pointed to a ref, so that the
Gerrit admin can take the appropriate actions to allow the replication
shortcut and avoid a full git fetch.

Change-Id: I7dbaa7b6ed35c389c7e2477467829d85dfc542e8