Bug#601859: wesnoth-1.9: Bad error handling in postinst script

[PJ Weisberg]

As a temporary workaround, until the postinst script gets fixed, you can:

1. Install wesnoth-1.8-core
2. Install wesnoth-1.9-core
3. Uninstall wesnoth-1.8-core

The  Wesnoth1.9 postinst configuration script is set up to abort if
Wesnoth isn't already configured, so installing a version that does
successfully configure tricks it into working.