Support structured errors for GetTreeEntries

What does this MR do and why?

Gitaly is in the process of converting to structured errors for the GetTreeEntries RPC. This allows for more details errors to be returned to rails.

Update #tree_entries to handle structed errors.

To do this, we also update to the latest gitaly gem.

Issue: Introduce structured error definitions for GetT... (gitaly#5036 - closed)
Gitaly MR (blocked on this MR): commit: Use structured errors in `GetTreeEntrie... (gitaly!5895 - merged)

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