Use res.message for sending batch file errors.

The messenger middleware provides this method.

Dav recently fixed a bug for Chrome by sending
a body for 404 errors. Since this method will also
send a body, the bug should remain fixed.

lib/hub/batch.js

@@ -181,22 +181,15 @@ Batch.prototype.handleFileRequest = function (server, filename) {
                     server.res.writeHead(302, {
                         "Location": agent.nextURL()
                     });
+                    server.res.end();
                 } else {
                     batch.report("agentError", agent, {
                         message: "Unable to serve the file: " + filename + ", ignoring"
                     });
-                    server.res.writeHead(404, {
-                        "content-type": "text/plain"
-                    });
-                    server.res.end("Not Found");
+                    server.res.message(404);
                 }
-
-                server.res.end();
             } else {
-                server.res.writeHead(500, {
-                    "content-type": "text/plain"
-                });
-                server.res.end("Unable to serve this file.");
+                server.res.message(500);
             }
             return;
         }