I've seen this in other packages such as mysql. Admittedly, it can be a bit frustrating that an inability to cleanly update/install a package can cause issues during an upgrade.
However, it is possible that something else may depend on a package cleanly upgrading before it can proceed. I can think of a few cases where this could happen. To be perfectly honest, I'd like to make an inability to start not prevent a clean upgrade as well. We'll definitely need to discuss it a bit. I do wish there was a Debian policy about this because we've really just stuck with how most packages behave.
