Moving decode_detailed_error

decode_detailed_error is useful for more than just operation_service.
Move this method one level up to GitalyClient so other services can take
advantage of it.

Move new_detailed_error to spec helper

new_detailed_error is a useful helper for specs other than just
operations_service_spec. Move this into its own spec helper.

Add new git errors

Add two new git error types. We will utilize these in a future commit
that parses a gRPC detailed error and throws an appropriate error on the
Rails side.

Teach delete_refs to parse gRPC detailed errors

Give delete_refs the ability to catch a gRPC error and parse the
detailed error if there is one.