spec: ListVolumes-specific error types/codes needed

[akutz]

There is no ListVolumesError message and so currently the only way to return error information from the RPC call Controller.ListVolumes is to use a GeneralError message:

  message GeneralError {
    enum GeneralErrorCode {
      // Default value for backwards compatibility. SHOULD NOT be
      // returned by Plugins. However, if a Plugin returns a
      // `GeneralErrorCode` code that an older CSI client is not aware
      // of, the client will see this code (the default fallback).
      //
      // Recovery behavior: Caller SHOULD consider updating CSI client
      // to match Plugin CSI version.
      UNKNOWN = 0;

      // Indicates that an undefined error occurred. More human-readable
      // information MAY be provided in the `error_description` field.
      // The `caller_must_not_retry` field MUST be set appropriately by
      // the Plugin to provide callers expected recovery behavior.
      //
      // Recovery behavior: Caller MAY retry (with exponential backoff),
      // if `caller_must_not_retry` is set to false. Otherwise, the
      // caller MUST not reissue the same request.
      UNDEFINED = 1;

      // Indicates that the version specified in the request is not
      // supported by the Plugin. The `caller_must_not_retry` field MUST
      // be set to true.
      //
      // Recovery behavior: Caller MUST NOT retry; caller SHOULD call
      // `GetPluginInfo` to discover which CSI versions the Plugin
      // supports.
      UNSUPPORTED_REQUEST_VERSION = 2;

      // Indicates that a required field is missing from the request.
      // More human-readable information MAY be provided in the
      // `error_description` field. The `caller_must_not_retry` field
      // MUST be set to true.
      //
      // Recovery behavior: Caller MUST fix the request by adding the
      // missing required field before retrying.
      MISSING_REQUIRED_FIELD = 3;
    }

The above error codes do not provide a meaningful response to the following Controller.ListVolumes error cases:

• The starting_token value may be invalid.
• If max_entries is greater than zero then a plug-in may need to return an error to indicate that it does not support pagination.

[chhsia0]

We should also have a CALL_NOT_IMPLEMENTED error code for plugins without the RPC::LIST_VOLUMES and RPC::GET_CAPACITY capabilities.

[jdef]

pretty sure these concerns are addressed by #115. please re-open if needed