Update gNMI specification to clarify `Set` error handling.

[robshakir]

  * (M) rpc/gnmi/gnmi-specification.md
    - remove references to the deprecated `error` field in the
      `SetResponse` and `UpdateResult` messages, explicitly
      specifying the error handling behaviour follows the standard
      gRPC error model.

Fixes #62.

[dplore]

In the case of multiple errors, I believe we should specify "the implementation SHOULD return the first error encountered"

As a newcomer to gRPC, I found it useful to directly reference the standard gRPC return codes and this gRPC core doc page on status codes