Ensure mergeprop instantiation failures are not fatal

Instead of re-raising on a failure to create a MergeProp instantiation
only log. This prevents a failure (due to malformed review data) from
causing the reviewday report to become stale.

Change-Id: Ife8d11579004564547402260514caa012ec7fa4a
Closes-Bug: 1321015
Merge "Ensure mergeprop instantiation failures are not fatal"