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/cssom/lib/StyleSheet.js

@@ -0,0 +1,17 @@
+//.CommonJS
+var CSSOM = {};
+///CommonJS
+
+
+/**
+ * @constructor
+ * @see http://dev.w3.org/csswg/cssom/#the-stylesheet-interface
+ */
+CSSOM.StyleSheet = function StyleSheet() {
+	this.parentStyleSheet = null;
+};
+
+
+//.CommonJS
+exports.StyleSheet = CSSOM.StyleSheet;
+///CommonJS