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/fontFamily.js

@@ -0,0 +1,33 @@
+'use strict';
+
+var TYPES = require('../parsers').TYPES;
+var valueType = require('../parsers').valueType;
+
+var partsRegEx = /\s*,\s*/;
+module.exports.isValid = function isValid(v) {
+  if (v === '' || v === null) {
+    return true;
+  }
+  var parts = v.split(partsRegEx);
+  var len = parts.length;
+  var i;
+  var type;
+  for (i = 0; i < len; i++) {
+    type = valueType(parts[i]);
+    if (type === TYPES.STRING || type === TYPES.KEYWORD) {
+      return true;
+    }
+  }
+  return false;
+};
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('font-family', v);
+  },
+  get: function() {
+    return this.getPropertyValue('font-family');
+  },
+  enumerable: true,
+  configurable: true,
+};