Update sqitch verify command used during upgrade

We try to run sqitch verify to determine if sqitch is on a box during
upgrade. With the recent lock down of postgres, this needs to be run as
the db_user or else it will fail with an error, making the upgrade
process think sqitch isn't on the box, when it is.

Added a comment about this issue in the code as well to clarify this
issue.

Also removed 2 from the possible return values, as verify won't return a
2 (that's an issue with the sqitch deploy command in some cases).
If epmd isn't running, continue with upgrade, as that is desired state.

Previously, if epmd wasn't running, the upgrade would fail trying to
stop epmd. Since the desired state is that epmd not be running, we don't
care how it got that way, just move on with life.