Add null check to decrypt response

[nickrobison-usds]

When decryption fails due to an incorrect root key, the response from the decrypt function is null instead of false, this adds a check to ensure that the error is correctly thrown, otherwise the error comes from the downstream method and is less clear.

[hatched]

Thanks for the pull request @nickrobison-usds 👍 !

Could you please add an additional test case for this? The best place for it would be in https://github.com/go-macaroon/js-macaroon/blob/master/test/verify.js

You can run the tests locally with npm t.

[hatched]

Thanks again @nickrobison-usds