poll generates error with -vt=none

[i-keliukh]

Steps to reproduce

Run the following command:

python3.7 -m universum poll -vt=none

Expected result

Application finishes execution with error.

Actual result

Application produces the exception backtrace:

==> Universum 0.18.7 started execution

1. Enumerating changes
   └ [Success]

2. Finalizing
   └ [Success]

Traceback (most recent call last):
  File "/home/<user>/Projects/Universum/universum/lib/utils.py", line 182, in excepted_function
    func(*args, **kwargs)
  File "/home/<user>/Projects/Universum/universum/lib/utils.py", line 205, in function_in_block
    return self.structure.run_in_block(func, block_name, pass_errors, self, *args, **kwargs)
  File "/home/<user>/Projects/Universum/universum/modules/structure_handler.py", line 116, in run_in_block
    result = operation(*args, **kwargs)
  File "/home/<user>/Projects/Universum/universum/poll.py", line 65, in execute
    self.latest_cls = self.vcs.driver.get_changes(self.stored_cls, self.settings.max_number)
  File "/home/<user>/Projects/Universum/universum/modules/vcs/base_vcs.py", line 95, in get_changes
    raise NotImplementedError
NotImplementedError
==> Universum 0.18.7 finished execution