rethrow upstream HTTP errors

If the proxied request to the CDN fails,
rethrow the error.

We were ignoring errors and sending an empty body.

IPython/nbconvert/postprocessors/serve.py

@@ -1,16 +1,9 b''
 """PostProcessor for serving reveal.js HTML slideshows."""
-from __future__ import print_function
-#-----------------------------------------------------------------------------
-#Copyright (c) 2013, the IPython Development Team.
-#
+
+# Copyright (c) IPython Development Team.
 #Distributed under the terms of the Modified BSD License.
-#
-#The full license is in the file COPYING.txt, distributed with this software.
-#-----------------------------------------------------------------------------
 
-#-----------------------------------------------------------------------------
-# Imports
-#-----------------------------------------------------------------------------
+from __future__ import print_function
 
 import os
 import webbrowser
@@ -22,9 +15,6 b' from IPython.utils.traitlets import Bool, Unicode, Int'
 
 from .base import PostProcessorBase
 
-#-----------------------------------------------------------------------------
-# Classes
-#-----------------------------------------------------------------------------
 
 class ProxyHandler(web.RequestHandler):
     """handler the proxies requests from a local prefix to a CDN"""
@@ -37,12 +27,15 b' class ProxyHandler(web.RequestHandler):'
     
     def finish_get(self, response):
         """finish the request"""
-        # copy potentially relevant headers
+        # rethrow errors
+        response.rethrow()
+        
         for header in ["Content-Type", "Cache-Control", "Date", "Last-Modified", "Expires"]:
             if header in response.headers:
                 self.set_header(header, response.headers[header])
         self.finish(response.body)
 
+
 class ServePostProcessor(PostProcessorBase):
     """Post processor designed to serve files