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

commit-queue/buildbot_json.py

Index: commit-queue/buildbot_json.py
===================================================================
--- commit-queue/buildbot_json.py	(revision 237637)
+++ commit-queue/buildbot_json.py	(working copy)
@@ -946,9 +946,14 @@
     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: