Re-add Missing Error class

[shundhammer]

Bugzilla

https://bugzilla.suse.com/show_bug.cgi?id=1227580

Problem

yast2-services-manager fails to build in its unit tests with the latest systemd V256.

Cause

A very rare error condition was met, and it turned out that a refactoring in August 2018, 6 years ago (!), removed an error class; an error that was raised only once in all the YaST code, so the unit tests didn't cover it.

That happens when the systemctl get-default command fails.

The Change that Caused This

e4ab100#diff-8ddc7418f9da5ef96888a536c77c9e4c3330414a4b9a14f4ad95830dd00547ae

part of #799

Fix

This only fixes the unit tests failing because of the Ruby NameError: It adds the missing error class again. This does not fix the part that this problem appears in the first place: That the systemctl get-default command fails.

Is that an intentional and well-documented change in the systemd behavior? Probably not. So the unit tests may still fail, but now it will give a much clearer error that includes what the systemctl command complained about.

[coveralls]

coverage: 41.889% (+0.002%) from 41.887%
when pulling 70c3bf9 on huha-sysctl-err
into b49143a on master.

[dgdavid]

LGTM, Thanks a lot!

[yast-bot]

✔️ Internal Jenkins job #231 successfully finished
✔️ Created OBS submit request #1186617