code: func NewError(code int, s string) error {
  	if code < 0 || code >= ErrCnt {
  		panic("no such error code!")
  	}
  	r := &Error{
  		err: s,
  	}
  	switch code {
  	case ErrNotFound, ErrIO, ErrUnauthorized, ErrInvalidValue, ErrDataOverflow, ErrNothingToReturn, ErrInvalidSignature, ErrNotSynced, ErrPeriodDepth, ErrCorruptData:
  		r.code = code
  	}
  	return r
  }
docs: // NewError creates a new Swarm feeds Error object with the specified code and custom error message