Error messages are displayed using the following methods:
* **zenity** parses pango markup and replaces escaped characters
* **kdialog** replaces (some) escaped characters
* **gtk-dialog.py** replaces `\n`
* **printf** interprets format strings and replaces escaped characters
* **echo** just displays the text
The error messages themself contain escaped characters and paths from variables.
This PR unifies the behavior by:
* Use **printf** to format error messages and replace escaped characters
* Setting `--no-markup` for **zenity** to disable pango markup and escaped characters
* Remove `\n` replacement from **gtk-dialog.py**.
* Use plain **echo** instead of **printf**
This removes the "View FAQ" button, which does not work in the AppImage
enivronment. It also replaces the inconsistent question mark icon with
a more appropriate stop symbol, and makes the wording consistent across
the zenity, terminal, and native macOS dialogs.
