* Exceptions.cs: Add new exception, make UrlNotHtml a WebException
  and rename JsonException to something more specific.

* WeatherUnderground.cs: Deal with the situation that the server
  doesn't return any workable information.

* WebTools.cs: Fix YouTube duration extraction regexp.
Catch XML exceptions, in case the RSS feed gets corrupted in some way. This happened recently when Nyaa.se was offline for a few days.