Improve error handling for update-locale

Set absolute path to /usr/sbin/update-locale to make sure
the binary can be accessed no matter what's inside $PATH
and if it's not present inform user about it, but do not fail
because the locales-all package might be installed and it doesn't
provide the /usr/sbin/update-locale binary.

chroot-script

@@ -431,7 +431,13 @@ EOF
 # set default locales {{{
 default_locales() {
   if [ -n "$DEFAULT_LOCALES" ] ; then
-    update-locale LANG="$DEFAULT_LOCALES"
+    if ! [ -x /usr/sbin/update-locale ] ; then
+      echo "Warning: update-locale executable not available (no locales package installed?)"
+      echo "Ignoring request to run update-locale for $DEFAULT_LOCALES therefore"
+      return 0
+    fi
+
+    /usr/sbin/update-locale LANG="$DEFAULT_LOCALES"
   fi
 }
 # }}}