Livereload: show message when Webview can't connect

If this plugin tries to load the URL provided by Cordova (via `<content src="http://...">`), but it can't connect, the Webview ends up spinning at the splash screen forever, giving no clue to the user what's going on.

This plugin should show a message that it can't connect.

ref: https://github.com/ionic-team/ionic-cli/issues/4047