CQ: print buildbot URL it's trying to access on errors

commit-queue/buildbot_json.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file at
 # http://src.chromium.org/viewvc/chrome/trunk/src/LICENSE
 # This file is NOT under GPL.
 
 """Queries buildbot through the json interface.
 """
 
@@ skipping 928 matching lines @@
         if remaining > datetime.timedelta(seconds=0):
           logging.debug('Sleeping for %ss' % remaining)
           time.sleep(remaining.seconds)
       self.last_fetch = datetime.datetime.utcnow()
     url = '%s/json/%s' % (self.url, suburl)
     if '?' in url:
       url += '&filter=1'
     else:
       url += '?filter=1'
     logging.debug('read(%s)' % suburl)
-    channel = urllib.urlopen(url)
-    data = channel.read()
+    try:
+      channel = urllib.urlopen(url)
+      data = channel.read()
+    except IOError as e:
+      logging.warning('caught %s while fetching "%s"; re-throwing' % (
+                      str(e), url))
+      raise
     try:
       return json.loads(data)
     except ValueError:
       if channel.getcode() >= 400:
         # Convert it into an HTTPError for easier processing.
         raise urllib2.HTTPError(
             url, channel.getcode(), '%s:\n%s' % (url, data), channel.headers,
             None)
       raise
 
@@ skipping 480 matching lines @@
       gen_usage(parser, args[0])
       return command(parser, args[1:])
 
   # Not a known command. Default to help.
   gen_usage(parser, 'help')
   return CMDhelp(parser, args)
 
 
 if __name__ == '__main__':
   sys.exit(main())