Fix missing import and typos

[knabar]

I ran into the following error:

Traceback (most recent call last):
  File "/usr/local/bin/duplicity", line 1525, in <module>
    with_tempdir(main)
  File "/usr/local/bin/duplicity", line 1519, in with_tempdir
    fn()
  File "/usr/local/bin/duplicity", line 1373, in main
    do_backup(action)
  File "/usr/local/bin/duplicity", line 1494, in do_backup
    full_backup(col_stats)
  File "/usr/local/bin/duplicity", line 563, in full_backup
    globals.backend)
  File "/usr/local/bin/duplicity", line 444, in write_multivol
    (tdp, dest_filename, vol_num)))
  File "/usr/local/lib/python2.7/dist-packages/duplicity/asyncscheduler.py", line 146, in schedule_task
    return self.__run_synchronously(fn, params)
  File "/usr/local/lib/python2.7/dist-packages/duplicity/asyncscheduler.py", line 172, in __run_synchronously
    ret = fn(*params)
  File "/usr/local/bin/duplicity", line 443, in <lambda>
    async_waiters.append(io_scheduler.schedule_task(lambda tdp, dest_filename, vol_num: put(tdp, dest_filename, vol_num),
  File "/usr/local/bin/duplicity", line 335, in put
    backend.put(tdp, dest_filename)
  File "/usr/local/lib/python2.7/dist-packages/duplicity/backend.py", line 374, in inner_retry
    code = _get_code_from_exception(self.backend, operation, e)
  File "/usr/local/lib/python2.7/dist-packages/duplicity/backend.py", line 346, in _get_code_from_exception
    return backend._error_code(operation, e) or log.ErrorCode.backend_error
  File "/usr/local/lib/python2.7/dist-packages/duplicity/backends/b2backend.py", line 180, in _error_code
    return log.ErrorCode.backed_error
NameError: global name 'log' is not defined

Also noticed that backend... was spelled backed....

[mtbentley]

Looks good. Thanks