ui: Fix formatting of dev_server errors

Change-Id: Ida595863f5505f8c259cec52dad2c59cfe6607ba
diff --git a/tools/dev_server b/tools/dev_server
index bd74df7..fb836b8 100755
--- a/tools/dev_server
+++ b/tools/dev_server
@@ -68,6 +68,7 @@
           self.send_response(200)
           self.send_header("Content-type", "text/html")
           self.end_headers()
+          self.wfile.write("<pre>")
           self.wfile.write(e.stdout_and_stderr)
           return
         return SimpleHTTPRequestHandler.do_GET(self)