Add a failure message if $ruby_build_dir is missing.

Author: postmodern

bin/ruby-install

@@ -45,7 +45,7 @@ fi
 
 download_patches || fail "Fetching patches $patches failed!"
 apply_patches  || fail "Patching $ruby $ruby_version failed!"
-cd "$ruby_build_dir"
+cd "$ruby_build_dir" || fail "ruby build directory is missing: $ruby_build_dir"
 configure_ruby || fail "Configuration of $ruby $ruby_version failed!"
 clean_ruby     || fail "Cleaning $ruby $ruby_version failed!"
 compile_ruby   || fail "Compiling $ruby $ruby_version failed!"