Log failure in internalSend (elastic#131602)

An exception here should be impossible, but we don't assert that, nor do
we emit a log message to prove it didn't happen in a production
environment. This commit adds the missing log and assert.

Backport of elastic#131418 to `8.18`

Author: DaveCTurner

server/src/main/java/org/elasticsearch/transport/OutboundHandler.java

@@ -18,6 +18,7 @@
 import org.elasticsearch.TransportVersions;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.RecyclerBytesStreamOutput;
 import org.elasticsearch.common.network.CloseableChannel;
@@ -293,6 +294,16 @@ private void maybeLogSlowMessage(boolean success) {
                 }
             });
         } catch (RuntimeException ex) {
+            logger.error(
+                Strings.format(
+                    "unexpected exception calling sendMessage for transport message [%s] of size [%d] on [%s]",
+                    message,
+                    messageSize,
+                    channel
+                ),
+                ex
+            );
+            assert Thread.currentThread().getName().startsWith("TEST-") : ex;
             Releasables.closeExpectNoException(() -> listener.onFailure(ex), () -> CloseableChannel.closeChannel(channel));
             throw ex;
         }