Overlayfs update: fallback to a writable transaction-in-progress after a failed sync

This adds an attempt to remount rw the overlayfs mount after a failed sync attempt (e.g. in case only some members of a hardlink group were modified, which is not supported on overlayfs), which should result in a transaction in the same state it was in before the attempt at publish.  The user can then fix any hardlink issues and attempt the publish again. 

Also updated the error message given when the SyncUnionOverlayfs code detects hardlink issues, making the explanation language more clear and adding some whitespace to make the proposed commands to correct the issues stand out better.