A person‘s conduct is excused if he believes that the facts are such that his conduct is necessary and appropriate for any of the purposes which would establish a justification or excuse under this chapter, even though his belief is mistaken. However, if his belief is negligently or recklessly held, it is not an excuse in a prosecution for an offense for which negligence or recklessness, as the case may be, suffices to establish culpability. Excuse under    this section is a defense or affirmative defense according to which type of defense would be established had the facts been as the person believed them to be.