The masked error is returned from the url openers

OS: Ubuntu 14.04 LTS
APP: MaaS trusty

When using maas to import the images, it uses the simplestream to open the url.

The maas server is behind a proxy which only supports http but not https.
The generic error is reported. And the error is masking by the url opener.

root@yankai-maas-server-trusty:/var/log/maas# http_proxy=http://91.189.91.13:80 maas-import-pxe-files
2014-05-07 00:09:06,646 INFO Importing boot resources.
2014-05-07 00:09:06,678 ERROR Unhandled exception; unable to continue.
Traceback (most recent call last):
  File "/usr/sbin/maas-import-pxe-files", line 30, in <module>
    main(args)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/import_images/boot_resources.py", line 493, in main
    repo_boot = dumper.dump(source['path'], keyring=source['keyring'])
  File "/usr/lib/python2.7/dist-packages/provisioningserver/import_images/boot_resources.py", line 276, in dump
    super(RepoDumper, self).sync(reader, rpath)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 82, in sync
    content, payload = reader.read_json(path)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 39, in read_json
    raw = self.source(path).read().decode('utf-8')
  File "/usr/lib/python2.7/dist-packages/simplestreams/contentsource.py", line 143, in read
    self.open()
  File "/usr/lib/python2.7/dist-packages/simplestreams/contentsource.py", line 139, in open
    self.fd = self._open()
  File "/usr/lib/python2.7/dist-packages/simplestreams/contentsource.py", line 135, in _open
    raise myerr
IOError: Unable to open http://maas.ubuntu.com/images/ephemeral-v2/releases/streams/v1/index.sjson. mirrors=[]