fixed Zend\Log\Logger::registerErrorHandler() doesn't log previous exceptions

This PR logs each exception as a single message of a list of exceptions structured as previous exceptions.

Additions:
+ Map severity of instances of `ErrorException` into a priority like the error handler does
+ added the first test for the exceptions handler

**Info for merging into 2.1:**
The internal array `$errorPriorityMap` should be replaced with the static `Logger::$errorPriorityMap`