/usr/share/unattended-upgrades/unattended-upgrade-shutdown:apt_pkg.Error:/usr/share/unattended-upgrades/unattended-upgrade-shutdown@399:main

[Impact]

 * Unattended-upgrades-shutdown service fails to start printing a crash
 * The fix would not warrant an SRU itself but it is part of the full backport SRU tracked in LP: #1702793.

[Test Case]

 * Buggy version:
root@x2:~# echo extra-junk >> /etc/apt/apt.conf.d/50unattended-upgrades
root@x2:~# /usr/share/unattended-upgrades/unattended-upgrade-shutdown
Traceback (most recent call last):
  File "/usr/share/unattended-upgrades/unattended-upgrade-shutdown", line 119, in <module>
    apt_pkg.init_config()
SystemError: E:Syntax error /etc/apt/apt.conf.d/50unattended-upgrades:67: Extra junk at end of file
root@x2:~#

 * Fixed version:
# /usr/share/unattended-upgrades/unattended-upgrade-shutdown ; echo $?
1
root@bb-lp-1806076:~# tail /var/log/unattended-upgrades/unattended-upgrades-shutdown.log
2018-11-30 18:05:56,515 ERROR - Apt returned an error, exiting
2018-11-30 18:05:56,515 ERROR - error message: 'E:Syntax error /etc/apt/apt.conf.d/50unattended-upgrades:93: Extra junk at end of file'

[Regression Potential]

 * U-u-s used to exit crashing, now it exits with an error code, just logging the error. There is little chance for regressing here, but users may have harder time to figure out why u-u-s exited, they need to check the log file.
 * There is also an enhancement request tracked to start u-u-s with a default config when parsing the config fails to be able to act on shutdown:
https://github.com/mvo5/unattended-upgrades/issues/161

[Original Bug Text]

The Ubuntu Error Tracker has been receiving reports about a problem regarding unattended-upgrades. This problem was most recently seen with package version 1.5ubuntu3.18.10.1, the problem page at https://errors.ubuntu.com/problem/994a64ac5b092229a3b9f45045daa8e51371085b contains more details, including versions of packages affected, stacktrace or traceback, and individual crash reports.
If you do not have access to the Ubuntu Error Tracker and are a software developer, you can request it at http://forms.canonical.com/reports/.

The crash was first found and fixed in LP: #1737717 but the fix LP: #1778219 introduced it in a slightly different way.
With the fix for LP: #1737717 u-u.service can gracefully stop u-u when the APT configuration became broken _after_ starting the u-u.service but the service still throws an unhandled exception when it starts with an already broken APT configuration.