Improve exceptions system

[Spikhalskiy]

A current set of possible exceptions which you can get in AsyncCompletionHandler#onThrowable is unclear. It's not explained or enumerated in javadocs. Would be great to:

1. Improve exceptions hierarchy. For example, it's very uncomfortable to get java.io.IOException: Too many connections per host. Theoretically, I can recover from this situation and I want to put this recovery block to catch(TooManyConnectionsPerHostExceptions){} block for example, but I couldn't because we use general-purpose exceptions.

2. Add javadocs to the onThrowable method with list of possible custom exceptions and reasons.