Structured error messages
Starting with upstream v14.3, error messages have being added to the Gitaly protocol and this ongoing at 15.0. Most seem to be related to write operations, hence not immediately relevant for HGitaly. This seems to be important with Praefect, to avoid unneeded replications.
We need to investigate how they are supposed to work, and what the relation with gRPC errors is, and possibly implement a few.