Prevent clearing of channel groups when Addon is started while DVBLink server cannot be connected

When the DVBLink addon is started, it will try to connect to the DVBLink server and load the channels from the server. If the server cannot be reached, no channels can be loaded, obviously, causing the XBMC channel groups to be cleared, because non-existent channels will be deleted. 

This pull request will prevent the addon from starting when the DVBLink server cannot be reached. XBMC will keep trying to connect automatically as long as the addon is enabled.