spec: ValidateVolumeCapabilities has incorrect error codes

[akutz]

The RPC ValidateVolumeCapabilities was updated to accept a volume ID string (765a1a5) and volume attributes map (00095b2) instead of a VolumeInfo object. However, the RPC's error codes still includes the following:

      // Indicates that the specified `VolumeInfo` is not allowed or
      // understood by the Plugin. More human-readable information MAY
      // be provided in the `error_description` field.
      //
      // Recovery behavior: Caller MUST fix the `VolumeInfo` before
      // retrying.
      INVALID_VOLUME_INFO = 5;

Instead the RPC's error codes should refactor INVALID_VOLUME_INFO into INVALID_VOLUME_ID:

      // Indicates that the specified volume ID is not allowed or
      // understood by the Plugin. More human-readable information MAY
      // be provided in the `error_description` field.
      //
      // Recovery behavior: Caller MUST fix the volume ID before
      // retrying.
      INVALID_VOLUME_ID = 5;

[jdef]

resolved by #115. please re-open if needed