Show error if keyserver is not reachable

This pull request is added to fix #244

@dschuermann this is a first impression for you what I'm doing. This is not final, not completely tested and there is some debug stuff in it. But I wanted to ask you if this is what you wanted me to do. 

Several questions:
1) Is the concept which I'm trying to implement right?
2) Should we transfer exceptions or custom Strings etc. as error in the AsyncWrapper

Please give me some feedback.