feat: specific error for not found attribute

verify.go

@@ -350,12 +350,20 @@ func getCertFromCertsByIssuerAndSerial(certs []*x509.Certificate, ias issuerAndS
 	return nil
 }
 
+type AttributeNotFoundError struct {
+	AttributeType asn1.ObjectIdentifier
+}
+
+func (a AttributeNotFoundError) Error() string {
+	return fmt.Sprintf("pkcs7: attribute %s does not exist", a.AttributeType)
+}
+
 func unmarshalAttribute(attrs []attribute, attributeType asn1.ObjectIdentifier, out interface{}) error {
 	for _, attr := range attrs {
 		if attr.Type.Equal(attributeType) {
 			_, err := asn1.Unmarshal(attr.Value.Bytes, out)
 			return err
 		}
 	}
-	return errors.New("pkcs7: attribute type not in attributes")
+	return AttributeNotFoundError{AttributeType: attributeType}
 }