Fix bug: undefined variable used in logging call

[jdlangs]

Reference the issue numbers and reviewers
Bug introduced in PR #88 (@danisla)

Explain relevant issues and how this pull request solves them
It looks like the logging statement used when xfconf-query fails had been copied from set_dpi(dpi) for set_cursor_size(size) but the dpi variable was not changed. So when xfconf-query didn't work properly in my setup, it triggered an exception and crashed the program.

Describe the changes in code and its dependencies and justify that they work as intended after testing
After fixing the variable name, I confirmed there is no crash and a proper logging statement is generated letting me know xfconf-query failed.

• I confirm that this pull request is relevant to the scope of this project. If you know that upstream projects are the cause of this problem, please file the pull request there.
• I confirm that this pull request has been tested thoroughly and to the best of my knowledge that additional unintended problems do not arise.
• I confirm that the style of the changed code conforms to the overall style of the project.
• I confirm that I have read other open and closed pull requests and that duplicates do not exist.
• I confirm that I have justified the need for this pull request and that the changes reflect the fix for the specified problem.
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[ehfd]

Looks like this was the issue for #110 that everyone overlooked?

Thank you very much for your help. Now I understand that GM uses my project :).

[jdlangs]

Thanks, we appreciate the great work!