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.
Prefer to avoid blanket `rescue` since it catches all `StandardError` - we should catch the specific exception that we expect to be raised when `config/secrets.yml` isn't present.