librarian failures cause poimport to mark imports as failed (rather than waiting a while and retrying)

When librarian is down, poimport script fails with a backtrace like:

2008-04-28 13:22:07 ERROR Got an unexpected exception while importing Lao (lo) translation of sw-frmdlg in Ubuntu Hardy package "openoffice.org"
Traceback (most recent call last):
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/launchpad/scripts/po_import.py", line 102, in run
    (mail_subject, mail_body) = (
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/launchpad/database/pofile.py", line 703, in importFromQueue
    import_file = librarian_client.getFileByAlias(
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/librarian/client.py", line 312, in getFileByAlias
    return _File(urllib2.urlopen(url))
  File "/usr/lib/python2.4/urllib2.py", line 130, in urlopen
    return _opener.open(url, data)
  File "/usr/lib/python2.4/urllib2.py", line 364, in open
    response = meth(req, response)
  File "/usr/lib/python2.4/urllib2.py", line 471, in http_response
    response = self.parent.error(
  File "/usr/lib/python2.4/urllib2.py", line 402, in error
    return self._call_chain(*args)
  File "/usr/lib/python2.4/urllib2.py", line 337, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.4/urllib2.py", line 480, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
HTTPError: HTTP Error 502: Bad Gateway

This produces that we set the status of that import to 'FAILED'. Instead, given that no imports will success with Librarian being down, we should stop the script so it can resume its work in a future execution once Librarian is back to normality.