Give better hint for checkbox questions created with question() in try_again message

[rossellhayes]

This PR extends #732 to apply to checkbox questions created by question() in addition to checkbox questions created by question_checkbox(). It does this by changing the default value of try_again in question() to NULL. If try_again is NULL, question() determines the default message based on the question type.

Closes #782.

[gadenbuie]

Can you explain a bit more about the decision to add a new argument?

[rossellhayes]

@gadenbuie Because question() can create both radio questions and checkbox questions, it needs to have two different default "try again" messages: one for radio questions and one for checkbox questions.

An alternative solution would be to change the default value of try_again to NULL and determine the default within the function. Would you prefer that?

[gadenbuie]

An alternative solution would be to change the default value of try_again to NULL and determine the default within the function. Would you prefer that?

Yeah, that would be better. When you call question(), you're creating a unique question – only one try_again will apply.

[rossellhayes]

That makes sense, refactored in 316d85f

[gadenbuie]

Look great, thanks!