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.
When we change a global variable then set it back, we use `begin ... ensure ... end` to guarantee the verifier is set back to the original value even if an exception is raised.