fix: handle underflow error

[pete-eiger]

Description

The line let message_age = Utc::now().timestamp() as u64 - self.nonce; was panicking if the message nonce is "from the future", this update handles that case by returning Err and skipping the message.

Issue link (if applicable)

graphops/subgraph-radio#135

[hopeyen]

That's so weird, why would a message come from the future?

Alternatively if timestamp can be typed as u64, it might be cleaner to convert self.nonce to i64 when subtracting since there's the 0..LIMIT check. other than that, looks good!