Have rkv::error::StoreError implement std::error::Error

[rnewman]

This is a straightforward omission.

[mhammond]

rkv is now using thiserror so I don't think this is relevant any more? (In particular, the advice at https://github.com/mozilla/nimbus-sdk/blob/da8d27cdba0d908befbfc99af1b5ef5b5f042d1e/nimbus/src/error.rs#L56 now works with 0.17.0)