Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/cssstyle/lib/constants.js

@@ -0,0 +1,6 @@
+'use strict';
+
+module.exports.POSITION_AT_SHORTHAND = {
+  first: 0,
+  second: 1,
+};