Don't retry for PublicKeyNotFound

We only return `nil` when we know the person, but don't know the key or
the key is invalid, so it doesn't make sense to retry in this case. When
the person isn't known and can't be fetched we raise a DiscoveryError
which will be retried.

Also the errors were moved to the `Signable` module in the last release.

closes #7717