Enhance using cache if site is offline

We see thetvdb.com was offline the recent days.
This leads to a complete non-enhancement after import of new XMLTV, even though, information is cached.
Pls add a step, if thetvdb.com is offline, that GE uses the cached information.
Probably also valid for the case, if the LAN is down.

Importance: very low (doesn't happen often, hopefully) 
