If you have a bug that is blocked on something else, nag e-mails on the original bug don't help. It's blocked. At the same time, I don't want to add Disable-Nags on it, because it will be unblocked at some point.

Proposal:
* If a bug is eligible for nagging, but has open blockers, it won't get nagged.
* Possibly nag the blocker instead. (Caveat around blocks that block many things)
* Once the blocker is closed, nag pretty much immediately.