Should warn user when URL is not configured

[lapo-luchini]

Right now if you install the extension, don't configure the Pinry URL and just try and use it, it actualyl opens a script to "undefined" (or the not existing anymore official demo website) so visually nothing happens.
An error pop-up or something would be useful for the user to understand what's happening, maybe (if possible) complete with link to the options page to check the address.