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.
Cool. What about calling `.parse`?

It may be simpler to just let it raise a `NoMethodError` on the `nil` verifier.

Otherwise we need to dig deeper than this and accept a `verifier` option to create/parse/new, use `SignedGlobalID.verifier` as a default, then raise if `verifier` isn't set.

What do you think?