commit: Don't put tree entry name into error message
When checking our production logs for gitlab.com, the top-most error
message we see is that a specific tree entry was not found in the
TreeEntry()
RPC. This error includes the path of the tree entry that
wasn't found, which makes it impossible to deduplicate this error in
Sentry.
Convert the error to use structured metadata instead to fix this. This also serves as a test balloon to verify that we indeed propagate the gRPC error metadata as expected in production systems.
Changelog: fixed