Magento responding '503 service unavailable' fails the job

[Guewen Baconnier @ Camptocamp]

A job receiving a 503 service unavailable from the Magento API fails, it should be retried later instead.

Traceback (most recent call last):
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/connector/connector/queue/worker.py", line 122, in run_job
    job.perform(session)
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/connector/connector/queue/job.py", line 460, in perform
    self.result = self.func(session, *self.args, **self.kwargs)
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/magentoerpconnect/magentoerpconnect/unit/import_synchronizer.py", line 311, in import_record
    importer.run(magento_id, force=force)
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/magentoerpconnect/magentoerpconnect/unit/import_synchronizer.py", line 145, in run
    self.magento_record = self._get_magento_data()
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/magentoerpconnect/magentoerpconnect/unit/import_synchronizer.py", line 61, in _get_magento_data
    return self.backend_adapter.read(self.magento_id)
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/magentoerpconnect/magentoerpconnect/unit/backend_adapter.py", line 159, in read
    [int(id), attributes])
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/magentoerpconnect/magentoerpconnect/partner.py", line 190, in _call
    return super(PartnerAdapter, self)._call(method, arguments)
  File "/home/gbaconnier/code/dev_instances/openerp_magento7/parts/magentoerpconnect/magentoerpconnect/unit/backend_adapter.py", line 126, in _call
    self.magento.password) as api:
  File "/home/gbaconnier/.dotfiles/buildout/eggs/magento-0.1.4-py2.6.egg/magento/api.py", line 138, in __enter__
    self.username, self.password)
  File "/usr/lib/python2.6/xmlrpclib.py", line 1199, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib/python2.6/xmlrpclib.py", line 1489, in __request
    verbose=self.__verbose
  File "/usr/lib/python2.6/xmlrpclib.py", line 1243, in request
    headers
ProtocolError: <ProtocolError for yushan.wrk.lsn:9100/index.php/api/xmlrpc: 503 Service Unavailable>