This sets a property on the callback object rather than passing an argument
to handleCompletion, to avoid accidentally breaking consumers who don't
expect arguments. The downside is that this required more changes to C++
consumers, but we control all of those so that seemed an acceptable
trade-off.
We should probably actually report errors for all the migrators, but I didn't
want to add risk (what if in some edge-case there are lots?) so I didn't. I'll
file a followup to update them to Cu.reportError() any errors.
MozReview-Commit-ID: Hue9Ci3hyVz
e1717a1338