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/properties/fontStyle.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var valid_styles = ['normal', 'italic', 'oblique', 'inherit'];
+
+module.exports.isValid = function(v) {
+  return valid_styles.indexOf(v.toLowerCase()) !== -1;
+};
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('font-style', v);
+  },
+  get: function() {
+    return this.getPropertyValue('font-style');
+  },
+  enumerable: true,
+  configurable: true,
+};