Comment 10 for bug 1951399

If I understand things correctly, then there are two common errors in logs for Budgie (tracked at bug 1950809), MATE and Xubuntu:

```
xubuntu plugininstall.py: log-output -t ubiquity chroot /target py3compile -p python3 /usr/share/python3/
xubuntu plugininstall.py: log-output -t ubiquity chroot /target mount -t proc proc /proc
xubuntu plugininstall.py: log-output -t ubiquity chroot /target mount -t sysfs sysfs /sys
xubuntu plugininstall.py: log-output -t ubiquity mount --bind /dev /target/dev
xubuntu plugininstall.py: log-output -t ubiquity mount --bind /run /target/run
--> xubuntu ubiquity: /usr/lib/python3/dist-packages/Onboard/LayoutLoaderSVG.py:447: SyntaxWarning: 'str' object is not callable; perhaps you missed a comma?
--> xubuntu ubiquity: raise Exceptions.LayoutFileError("Unrecognized modifier %s in" \
```

about onboard on Xubuntu and MATE. Does not exist in Kubuntu. Reported as bug 1951534 .

And

```
xubuntu ubiquity: umount: /target/cdrom: no mount point specified.
xubuntu plugininstall.py: log-output -t ubiquity umount /target/cdrom
xubuntu plugininstall.py: log-output -t ubiquity mount --bind /cdrom /target/cdrom
xubuntu ubiquity: chroot: failed to run command ‘tempfile’
xubuntu ubiquity: : No such file or directory
xubuntu plugininstall.py: log-output -t ubiquity umount /target/cdrom
xubuntu ubiquity: grep: /target/etc/apt/sources.list: No such file or directory
xubuntu plugininstall.py: Exception during installation:
xubuntu plugininstall.py: Traceback (most recent call last):
xubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 1854, in <module>
xubuntu plugininstall.py: install.run()
xubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 60, in wrapper
xubuntu plugininstall.py: func(self)
xubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 188, in run
xubuntu plugininstall.py: self.configure_apt()
xubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 588, in configure_apt
xubuntu plugininstall.py: raise install_misc.InstallStepError(
xubuntu plugininstall.py: ubiquity.install_misc.InstallStepError: AptSetup failed with code 127
```

about third-party components installation by Ubiquity. Happens in Budgie, Kubuntu, Xubuntu and MATE.

The first looks non-fatal, but the second seems to be fatal and possibly leads to crash.

Waiting for reaction from official Ubiquity developers as soon as possible.