Propagate the merge failure code to SnapshotMergeStats. am: 6ac3d47472 am: 4f94f1c101

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1675999

Change-Id: Id8640f0e4b966f52e5c84816a49d3e1c4be56322