BQ Write: Share stream closing errors across pending writes. #7005

While gax::Error is not Clone and our retry policies accept them by value... tonic::Status is Clone.

So we can easily construct a gax::Error from it and pass that along to the N callers.

This gives us the best of both worlds:

• reuse familiar retry policies
• no synthetic, transient errors after a stream fails with a permanent error.

https://github.com/googleapis/google-cloud-rust/pull/6994/changes#r4108025829

Although, now I need to think... what if we are multiplexing, and a req to table 1 fails with PERMISSION_DENIED and then req to table 2 (which could have succeeded) get the same error. 🤔