Log stack traces for MultipleFailureException

When a MultipleFailureException is thrown, only the name and message
of each exception is logged, not the stack trace for each exception.
This makes debugging multiple failures very difficult. Let's log the
stack traces, and then rethrow, so it's clear what caused each error.
Merge pull request #284 from spotify/rculbertson/multi-failure-exception

Log stack traces for MultipleFailureException