Support RPC specific error details when generically unwrapping structured Gitaly errors
Gitaly often returns structured errors by appending details to the RPC error. These error details allow Gitaly clients to differentiate errors with the same status code that would otherwise be ambiguous. Rails has a generic Gitaly RPC error unwrapper that inspects the status code and error detail and maps the error to something the client can use/handle. This can be seen in Rails here.
Some RPCs in Gitaly have RPC specific error detail types that wrap the set of details that the RPC is capable of returning (example). When this wrapper detail type is used, Rails is unable to generically decode the underlying detail and map to the correct error.
Update Rails so RPC error details can be generically mapped to the correct error type.