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/backgroundImage.js

@@ -0,0 +1,32 @@
+'use strict';
+
+var parsers = require('../parsers');
+
+var parse = function parse(v) {
+  var parsed = parsers.parseUrl(v);
+  if (parsed !== undefined) {
+    return parsed;
+  }
+  if (
+    parsers.valueType(v) === parsers.TYPES.KEYWORD &&
+    (v.toLowerCase() === 'none' || v.toLowerCase() === 'inherit')
+  ) {
+    return v;
+  }
+  return undefined;
+};
+
+module.exports.isValid = function isValid(v) {
+  return parse(v) !== undefined;
+};
+
+module.exports.definition = {
+  set: function(v) {
+    this._setProperty('background-image', parse(v));
+  },
+  get: function() {
+    return this.getPropertyValue('background-image');
+  },
+  enumerable: true,
+  configurable: true,
+};