Comment 7 for bug 1614576

Using errors='ignore' allows the upgrade to complete and doesn't seem too horrible given that's its non-interactive anyway.

               sys.stdout.write("%s" % s.decode(locale.getpreferredencoding(), errors='ignore'))

We can see the errors in the terminal output:

Preparing to unpack .../inputattach_1%3a1.5.1-2_amd64.deb ...
Unpacking inputattach (1:1.5.1-2) over (1:1.4.9-1) ...

Notice the ':' vs '%3a'. The same errors also appear in the log - /var/log/dist-upgrade/main.log.