TreeEntry: return gRPC NotFound when entry not found

Mikhail Mazurskiyrequested to merge
ash2k/tree-entry-not-found into master

This MR changes the TreeEntry API to return gRPC NotFound when entry was not found.

Current behavior is to return an empty response, which is not a great API. It's hard to use and led to a bug in GitLab Kubernetes Agent: gitlab-org/cluster-integration/gitlab-agent#88 (closed). See the MR for more details: gitlab-org/cluster-integration/gitlab-agent!267 (merged), specifically internal/gitaly/path_fetcher.go.

It's a breaking change for existing clients, so I guess it cannot be merged as is. I'm opening this MR anyway to start a conversation. Maybe there is a way forward still (e.g. a new API method?).

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