Improve the error when remapping to an invalid channel

Maps are proving to be a bit difficult to understand for some users.  It
is worth catching some of the common problems and more aggressively
reporting what might have gone wrong.

Change-Id: Ie9d5d62a8843a0d004da8eb4a18f6bbf83b47aa6
diff --git a/aos/configuration.cc b/aos/configuration.cc
index adba7dc..4c11405 100644
--- a/aos/configuration.cc
+++ b/aos/configuration.cc
@@ -641,6 +641,12 @@
   } else {
     VLOG(1) << "No match for { \"name\": \"" << name << "\", \"type\": \""
             << type << "\" }";
+    if (original_name != name) {
+      LOG(WARNING) << "Remapped from {\"name\": \"" << original_name
+                   << "\", \"type\": \"" << type << "\"}, to {\"name\": \""
+                   << name << "\", \"type\": \"" << type
+                   << "\"}, but no channel by that name exists.";
+    }
     return nullptr;
   }
 }