When we package Flutter, we used to find the "current" tag (which is the version number) by starting at the revision we are building on and looking backwards in time to find the most recent tag. This causes problems on release builds when we failed to tag properly.

This PR makes the packaging script be more strict by requiring the given revision to itself have a tag, but only when we're publishing the result. When we're not publishing the result, it's more lenient, since otherwise we couldn't test packaging on non-release commits.

I also renamed ProcessRunnerException to PreparePackageException, since we were using that exception more generally than just for processes.