Print channel which is missing matching data.

When we observe a timestamp without data, the first question is "which
channel?".  Add that info to the CHECK statement.

Change-Id: Ida1030ba00b196920a950ded18264618cd7d287e
diff --git a/aos/events/logging/logger.cc b/aos/events/logging/logger.cc
index 5a6bb8f..0aa37e7 100644
--- a/aos/events/logging/logger.cc
+++ b/aos/events/logging/logger.cc
@@ -1324,9 +1324,12 @@
            !state->at_end()) ||
           timestamped_message.data.span().size() != 0u) {
         CHECK_NE(timestamped_message.data.span().size(), 0u)
-            << ": Got a message without data.  Forwarding entry which was "
-               "not matched?  Use --skip_missing_forwarding_entries to "
-               "ignore this.";
+            << ": Got a message without data on channel "
+            << configuration::CleanedChannelToString(
+                   logged_configuration()->channels()->Get(
+                       timestamped_message.channel_index))
+            << ".  Forwarding entry which was not matched?  Use "
+               "--skip_missing_forwarding_entries to ignore this.";
 
         if (update_time) {
           // Confirm that the message was sent on the sending node before the