prf WalkerBase.walk can fail if self.open raises an exception

[Curtis Hovey]

The product release finder failed on the first use the an FTPWalker. This is probably an operational issue where the site or the port is blocked. Regardless of the connectivity issue, the PRF should not fail.

From the prf log:
2011-02-11 07:22:36 INFO Connecting to ftp.gnu.org
2011-02-11 07:22:36 ERROR Unhandled exception

From the tb in the librarian
  File "/srv/launchpad.net/production/launchpad-rev-12335/lib/lp/registry/scripts/productreleasefinder/walker.py", line 126, in walk
    self.open()
  File "/srv/launchpad.net/production/launchpad-rev-12335/lib/lp/registry/scripts/productreleasefinder/walker.py", line 206, in open
    self.ftp.connect(self.host)
  File "/usr/lib/python2.6/ftplib.py", line 131, in connect
    self.sock = socket.create_connection((self.host, self.port), self.timeout)
  File "/usr/lib/python2.6/socket.py", line 514, in create_connection
    raise error, msg
error: [Errno 111] Connection refused

[Launchpad QA Bot (lpqabot)]

Fixed in stable r12371 <http://bazaar.launchpad.net/~launchpad-pqm/launchpad/stable/revision/12371>.