Failed to connect up the `destroy' signal in error_box(), causing

Failed to connect up the `destroy' signal in error_box(), causing
gtk_main_quit not to get called, with the result that the whole game
ended up running one gtk_main level lower down. This meant that
final dialog box processing was not performed, so that (for example)
selecting 1x1 in Net, hitting OK and getting an error box, then
selecting a different size and hitting OK again failed to change to
the new size.

[originally from svn r4376]