Email is unclear when package is held back due to conffile prompt

[fragglet]

When a package has a conffile prompt requiring manual intervention, unattended-upgrades holds the package back (blacklists it). The email subject line looks like this:

Subject: [package on hold] unattended-upgrades result for zzzz: FAILURE

The email body then looks like this; in this example, linux-image-686-pae has been manually held back with apt-mark but netatalk is upgradable with a conffile prompt:

Unattended upgrade result: All upgrades installed

Packages that attempted to upgrade:
 freedoom gir1.2-rsvg-2.0 gstreamer1.0-plugins-good libhwloc15
 librsvg2-2 librsvg2-common netatalk python3-dateutil python3-mutagen
 qemu-system-common qemu-system-data qemu-system-gui qemu-system-misc
 qemu-system-modules-opengl qemu-system-ppc qemu-system-sparc
 qemu-system-x86 qemu-utils shared-mime-info

Packages with upgradable origin but kept back:
 Debian testing:
  linux-image-686-pae libgexiv2-2 netatalk

I have several criticisms of this email:

• The subject line says FAILURE, but confusingly the email then begins: Unattended upgrade result: All upgrades installed, so the email subject and body contradict each other. It is also not true: not all upgrades have been installed because some need to be manually upgraded.
• There is no clear statement made at the top of the file that manual intervention is needed.
• No distinction is made between packages that have been manually held back and those that require manual intervention; they are all lumped into the same category. The very bottom of the email lists the packages held back and says that the netatalk package is "blacklisted" - terminology that sounds like something I have manually chosen (and have not):

Package libgexiv2-2 is kept back because a related package is kept back or due to local apt_preferences(5).
Package linux-image-686-pae is marked to be held back.
Package netatalk is blacklisted.

• It is only towards the bottom of the email that the line Package netatalk has conffile prompt and needs to be upgraded manually appears. It's buried among some other text and doesn't stand out.

I suggest the following:

1. When manual intervention/upgrade is required, the subject line should begin [Action Required]. A good subject line might be something like: [Action Required]: Package foo requires manual upgrade
2. The email should not say "All upgrades installed" if not all upgrades have been installed.
3. Emails requesting manual intervention should probably be separate from those notifying about successful upgrades.
4. The email should be clear and actionable: something like:

Action is required to upgrade the following packages that could not be automatically upgraded:
  libgexiv2-2 netatalk

To manually upgrade them, type the following command as root:
  apt upgrade libgexiv2-2 netatalk