code: func CombineErrs(msg string, code ErrCode, errs ...error) error {
  	var nonNilErrs []error
  	for _, err := range errs {
  		if err != nil {
  			nonNilErrs = append(nonNilErrs, err)
  		}
  	}
  
  	switch len(nonNilErrs) {
  	case 0:
  		return nil
  	case 1:
  		return nonNilErrs[0]
  	default:
  		return WrapErrorf(multiError(nonNilErrs), code, "%s", msg)
  	}
  }
docs: // CombineErrs returns an error that wraps all the non-nil errors passed to it.
  // If all errors are nil, returns nil.
  // If there's only one non-nil error, returns that error without wrapping.
  // Else, returns an error with the message and code as passed, with the cause set to an error
  // that contains all the non-nil errors and for which Error() returns the concatenation of all their messages.