chromium-cpp: Set status code on errors, so that the console can track them.

BUG=none
R=inferno@chromium.org

Review URL: https://codereview.chromium.org/598063002

Cr-Commit-Position: refs/heads/master@{#296429}

styleguide/c++/chromium-cpp/main.py

@@ -33,6 +33,7 @@ class GitilesMirrorHandler(webapp2.RequestHandler):
         if not contents or self.request.get('bust'):
             result = urlfetch.fetch(url)
             if result.status_code != 200:
+                self.response.set_status(result.status_code)
                 self.response.write('http error %d' % result.status_code)
                 return
             contents = base64.b64decode(result.content)