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.
Agreed, not something we'd want to stick with. It's attractive as an intermediate step since it fixes Rails' tests and lets it move forward while we proceed with deeper improvement.

Makes sense to accept a verifier when creating, parsing, and initializing signed gids. (Imagine an API where `SignedGlobalID` wasn't available as a shared verifier)