Report unresolvable merge conflicts

[artwyman]

This is a follow-on to #1342.

The basic form of merge we've implemented for both PCDs and feeds is based on keeping all unique objects based on IDs (PCD ID, subscription ID, feed ID). There isn't currently any checking of the contents of a given PCD/subscription. If there are 2 different objects with the same ID, The structure of the merge implicitly prefers the "remote" copy, meaning the one which was on the server first. That has the potential to be wrong if a PCD is edited/replaced (rather than removed/added).

The first step in handling these situation is to detect it, and report it as part of the server event for merging conflicts. That will allow us to decide how often it happens, and how important it might be to fix.