ReplayMerge::doWork throws exceptions without descriptions.

[yeetfield]

At https://github.com/real-logic/aeron/blob/master/aeron-archive/src/main/cpp/client/ReplayMerge.h#L118 it seems like we're discarding the ex.what() present on the thrown exception, and it might be useful to propagate that so that callers of doWork have a useful error message. In my case, I was timing out after having specified an invalid replay destination endpoint and was trying to catch an aeron::util::TimeoutException before realizing that this specific type and its message was erased by doWork.