Improve error message

Summary:
DesktopToJson will convert lists such as "Something=a, b, c" into
JSON "Something": ["a", "b ", "c "]. Making sure there's quotes will lessen
this problem to some extent.

Test Plan:
Now the error message is:
```
Error: Installation of /home/apol/Downloads/kdeokktester/tester failed: Could not install dependency: ' kns://plasma-themes.knsrc/api.kde-look.org/1136471'
```

Reviewers: #plasma, mart

Subscribers: plasma-devel, #frameworks

Tags: #plasma, #frameworks

Differential Revision: https://phabricator.kde.org/D3953