Improve the error when you change the TimeConverter

We were exploding with a miss-matched UUID error when the problem was
that the time converter changed.  Instead, print out a more useful
error which is easier to fix.

Change-Id: Ie233e34f5de93d332d46e9e095231e2454431cd5
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/events/simulated_event_loop.cc b/aos/events/simulated_event_loop.cc
index 9121a8f..11e10a6 100644
--- a/aos/events/simulated_event_loop.cc
+++ b/aos/events/simulated_event_loop.cc
@@ -1275,7 +1275,10 @@
   scheduler_.ScheduleOnStartup([this]() {
     UUID next_uuid = scheduler_.boot_uuid();
     if (boot_uuid_ != next_uuid) {
-      CHECK_EQ(boot_uuid_, UUID::Zero());
+      CHECK_EQ(boot_uuid_, UUID::Zero())
+          << ": Boot UUID changed without restarting.  Did TimeConverter "
+             "change the boot UUID without signaling a restart, or did you "
+             "change TimeConverter?";
       boot_uuid_ = next_uuid;
     }
     VLOG(1) << scheduler_.distributed_now() << " " << NodeName(this->node())