Improve rescue of ZeroMQ discovery

This change is consist of  two parts.

1. The first one is about error message in case of `LoadError`.
 
 When I tried to figure out #96 I hadn't chance to understand what's going on. 

 The message of LoadError exception is really long, but have full view of discovery process. 

 I think it would be great if people will see full story of discovery process.

2. The second one is about double `map` execution.