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.

[wenovus]

LGTM

[wenovus]

@dplore noted here that there is ambiguity as to what to return when multiple error codes are valid: #155 (comment)

[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

[dplore]

Suggested change

	* When the client specifies a value which is not permitted by the target - `Unimplemented (12)`
	SHOULD be used. For example, setting an interface full_duplex leaf to false when the interface
	only supports full_duplex.

[dplore]

Please consider comments on

• additional use case for unimplemented (12) and 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

I found these more straightforward to read than the referenced gRPC error handling guide.