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**

As mentioned in OpenRA#20059 (comment)