Improve error message in invalid Sender

The normal Send messages (when you use MakeBuilder) already have good
checks, but the rarely-used copy-sender doesn't provide a good error
message.

Change-Id: Ie0f62aec7a90a01d5b4976a901e9ac03567a1302
Signed-off-by: James Kuszmaul <jabukuszmaul+collab@gmail.com>
diff --git a/aos/events/event_loop_tmpl.h b/aos/events/event_loop_tmpl.h
index 132da04..cc8e5c3 100644
--- a/aos/events/event_loop_tmpl.h
+++ b/aos/events/event_loop_tmpl.h
@@ -427,6 +427,7 @@
 template <typename T>
 RawSender::Error Sender<T>::Send(
     const NonSizePrefixedFlatbuffer<T> &flatbuffer) {
+  CHECK(valid()) << ": Sender must be initialized before sending.";
   return sender_->Send(flatbuffer.span().data(), flatbuffer.span().size());
 }