Raise error when try to initialize CaptchaTextInput alone and/or when try to initialize CaptchaField with widget keyword argument

First problem occurs if someone want to initialize CaptchaTextInput widget with some attributes like this:

    widget = CaptchaTextInput(attrs={'required': True})

And the second problem is when we have some custom widget or instance of CaptchaTextInput, and we want to pass it as keyword argument to CaptchaField like this:

    field = CaptchaField(widget=widget)