/**
  * Call the JavaScript error callback for this plugin.
  *
  * @param pluginResult The result to return.
  * @param callbackId The callback id used when calling back into JavaScript.
  */
 public void error(PluginResult pluginResult, String callbackId) {
   this.webView.sendJavascript(pluginResult.toErrorCallbackString(callbackId));
 }