Fail mock_callable / mock_constructor configuration if it already received a call

[fornellas]

By design, mock_callable() should be used like this:

self.mock_callable(target, 'attr')\
  .for_call(something)\
  .to_return_value(None)\
  .and_assert_called_once()
do_target_attr_call()

But sometimes people misuse it like:

mock = self.mock_callable(target, 'attr')\
  .for_call(something)\
  .to_return_value(None)
do_target_attr_call()
mock.and_assert_called_once()

Although this works right now, it is highly undesirable, as this undocumented usage pattern blocks implementation of #3, which can allow unexpected calls to fail on the spot, not after the test finishes.

Let's make mock_callable / mock_constructor fail if it is attempted to receive extra configuration, after it received the first call, which should prevent and educate users on this.