Unintuitive error message when failing to use LogReader::OnStart methods #20
Description
From @jkuszmaul
When we hit https://github.com/RealtimeRoboticsGroup/aos/blob/master/aos/events/simulated_event_loop.cc#L1463 it typically seems to be people creating EventLoops outside of the LogReader::OnStart. Confirm the nature of the issue here and improve the error message (and/or consider changing when EventLoop creation is even allowed) so that users know what to do when they see that error.