proto: Add structured error proto message definitions for FindCommits grpc

This MR adds new proto definition for the structured errors.

These errors will be used by FindCommits grpc when it can't find any commits (i.e. return a NotFound error) due to git-log(1) command has errors.

This MR was originally part of !6575 (closed). The old MR is refactored into 2 new MRs in order to unblock rail side. To be specific:

• This MR is merged to add proto definition.
• Another MR on rail side will use these proto definitions to fix rail tests related to FindCommits grpc.
• Another MR !6590 (merged) on gitaly side will be merged to let FindCommits grpc use the structured errors.