nginx: output config on failed gixy check (#389550)

pkgs/build-support/writers/scripts.nix

···
       ''
         # nginx-config-formatter has an error - https://github.com/1connect/nginx-config-formatter/issues/16
         awk -f ${awkFormatNginx} "$textPath" | sed '/^\s*$/d' > $out
-        gixy $out
+        gixy $out || (echo "\n\nThis can be caused by combining multiple incompatible services on the same hostname.\n\nFull merged config:\n\n"; cat $out; exit 1)
       '';
 
   /**