No-Root Improperly Handled

### Specifications
- Hardcode-Tray version: 4.3
- Distribution: Ubuntu 18.04 LTS
- Gtk version: 3.22
- Inkscape version: Inkscape 0.92.3 (2405546, 2018-03-11)
- Cairo version: 1.16.2-1
- Sni-qt version: 0.2.7

### Expected behavior and actual behavior
Run the script without root, gracefully complains. Actually just spews a bunch of Python errors. I'm not sure what has changed since I last accidentally did this (which probably when reporting for #397) but it definitely hasn't always behaved this way.

### Steps to reproduce the problem
1. Install hardcode-tray
2. Run `hardcode-tray -v` without root

Run `hardcode-tray -v` without root to capture the Python errors, then trace the command's entry point and existing privilege handling. The work is done when the command reports a clear, graceful error instead of emitting a traceback.

Written by the indexing model from the issue text.