Gracefully handle verification events that are unknown locally

[ulyssa]

Someone reported in #iamb-users that the client crashed with:

 thread 'iamb-worker-0' panicked at 'called `Option::unwrap()` on a `None` value', src/worker.rs:849:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This is failing in the handler for ToDeviceKeyVerificationRequestEvent, when it tries to unwrap the get_verification_request() return value. It should just ignore it if it's unknown locally.