Set config.global_id.verifier to nil if #message_verifier raises

Fixes #4.

Set `config.global_id.verifier = nil` if `app.message_verifier` raises due to missing secrets.yml. Ensure that SignedGlobalID raises if verifier is nil.
I definitely think being explicit about the error is worth it. While it is simpler to allow the `NoMethodError` to come through, I don't think it's the best experience.

We should probably require a verifier for `parse` as well, although allowing clients to pass a `verifier` option might be necessary there as well. How do you feel about that?