Comment 2 for bug 1766176

The solution seems to be re-issue the apt dist-upgrade command again. And this time the upgrade process will ended successfully with a Warning:

$ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
  linux-headers-4.15.0-15 linux-headers-4.15.0-15-generic linux-image-4.15.0-15-generic linux-image-extra-4.15.0-15-generic
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n]
Setting up initramfs-tools (0.130ubuntu3) ...
update-initramfs: deferring update (trigger activated)
Processing triggers for initramfs-tools (0.130ubuntu3) ...
update-initramfs: Generating /boot/initrd.img-4.15.0-19-generic
Using config file '/etc/zipl.conf'
Building bootmap in '/boot'
Building menu 'menu'
Adding #1: IPL section 'ubuntu' (default)
Adding #2: IPL section 'old'
Preparing boot device: vda (0000).
Done.
W: APT had planned for dpkg to do more than it reported back (0 vs 4).
   Affected packages: initramfs-tools:s390x
$ echo $?
0