Improve handling of ConnectionError

In a ConnectionError happens, try and capture the information for which resolver it occurred. While it may be just a regular one-off network blip, it may also indicate a failure mode in one of the target resolvers. That is especially true if the same connection error happens repeatedly for a single resolver.

Prior to this change, the information about the resolver was lacking. This made it hard to assess whether the connection errors were caused by some network issue, or if it's likely that one of the target resolver is at fault.