Continue if there is an old executable

Chances are that the previous build works just as well.
We just need it for config file parsing which does not change often.

data/tools/unit_tree/update-wmlunits

@@ -42,7 +42,8 @@ mkdir -p "$trans"
 # we only need the parser, but have to build the game executable for that
 echo BUILDING
 cd "$dir" || exit # the scons build places the wesnoth executable in the current directory
-TRAVIS=1 TRAVIS_OS_NAME=Linux scons -Y "$src" --option-cache="$build/.scons-option-cache"
+# continue on failure if there is an executable from a previous build
+TRAVIS=1 TRAVIS_OS_NAME=Linux scons -Y "$src" --option-cache="$build/.scons-option-cache" || [ -x wesnoth ]
 
 # later can just rebuild updated campaigns, but for now rebuild everything
 rm -f "$dir/overview.txt"