[5.2] Ensure encrypter exists

[greydnls]

This PR:

• declares encrypter as a proper property on the Queue class
• ensures that encrypter exists, or throws an exception if it doesn't.

Without that guard in place there's the possibility of getting "function encrypt called on null" error.

[GrahamCampbell]

Thanks, but this is actually a missuse of the encrypt exception.

[greydnls]

@GrahamCampbell I figured that might be the case. What would be a better Exception to use?

[GrahamCampbell]

I'd say just let it fatal error like it is now.