err variable needs to be initialized before assignment

This is to fix the following traceback:

Traceback (most recent call last):
  File "/usr/sbin/rhn_check", line 351, in __run_action
    (status, message, data) = CheckCli.__do_call(method, params, kwargs)
  File "/usr/sbin/rhn_check", line 344, in __do_call
    retval = method(*params, **kwargs)
  File "/usr/share/rhn/actions/errata.py", line 78, in update
    return packages.update(packagelist, cache_only)
  File "/usr/share/rhn/actions/packages.py", line 372, in update
    if err:
<type 'exceptions.UnboundLocalError'>: local variable 'err' referenced
before assignment