radicle: Fix `storage::refs::Error::is_not_found` — heartwood

Detection of errors of the “not found” kind does not account for the respective variant in sigrefs::read::error::Read. Fix this by adding it.

     pub fn is_not_found(&self) -> bool {
         match self {
             Self::Git(e) => e.is_not_found(),
             Self::Read(sigrefs::read::error::Read::MissingSigrefs { .. }) => true,
             _ => false,
         }
     }