Improve error messages in log_to_mcap

Change-Id: I6afca36a233a7e72f4dc32bbda7c383b2b45cfbb
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/util/log_to_mcap.cc b/aos/util/log_to_mcap.cc
index 8661246..11b7940 100644
--- a/aos/util/log_to_mcap.cc
+++ b/aos/util/log_to_mcap.cc
@@ -72,6 +72,10 @@
   if (FLAGS_include_clocks) {
     aos::logger::LogReader config_reader(logfiles);
 
+    if (aos::configuration::MultiNode(config_reader.configuration())) {
+      CHECK(!replay_node.empty()) << ": Must supply a --node.";
+    }
+
     const aos::Configuration *raw_config = config_reader.logged_configuration();
     config = aos::configuration::AddChannelToConfiguration(
         raw_config, "/clocks",
@@ -86,9 +90,12 @@
   aos::SimulatedEventLoopFactory factory(reader.configuration());
   reader.RegisterWithoutStarting(&factory);
 
+  if (aos::configuration::MultiNode(reader.configuration())) {
+    CHECK(!replay_node.empty()) << ": Must supply a --node.";
+  }
+
   const aos::Node *node =
-      (replay_node.empty() ||
-       !aos::configuration::MultiNode(reader.configuration()))
+      !aos::configuration::MultiNode(reader.configuration())
           ? nullptr
           : aos::configuration::GetNode(reader.configuration(), replay_node);