repository: Use `NOT_FOUND` gRPC code with `WriteRef` RPC
The WriteRef
RPC creates or updates a reference in a repository. If
the requested new or old revision objects do not exist in the
repository, the RPC returns an INTERNAL
gRPC error code.
Update the WriteRef
RPC error handling to instead return a NOT_FOUND
gRPC error code when the request contains objects revisions that do not
exist in the repository.
Related: #5711 (closed)
Edited by Justin Tobler