gemini: attempt mutation on both systems

EVen if mutation on the test system fails we now try to perform
the same mutation on the oracle as well.

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Gemini now tries to perform mutation on both systems regardless of
+  if one of them fail.
+
 ## [1.0.1] - 2019-05-16
 
 - Gemini ensures that primary key buckets do not overflow int32.

session.go

@@ -54,14 +54,14 @@ func (s *Session) Close() {
 	s.oracleSession.Close()
 }
 
-func (s *Session) Mutate(query string, values ...interface{}) error {
+func (s *Session) Mutate(query string, values ...interface{}) (e error) {
 	if err := s.testSession.Query(query, values...).Exec(); err != nil {
-		return fmt.Errorf("%v [cluster = test, query = '%s']", err, query)
+		e = multierr.Append(e, fmt.Errorf("%v [cluster = test, query = '%s']", err, query))
 	}
 	if err := s.oracleSession.Query(query, values...).Exec(); err != nil {
-		return fmt.Errorf("%v [cluster = oracle, query = '%s']", err, query)
+		e = multierr.Append(e, fmt.Errorf("%v [cluster = oracle, query = '%s']", err, query))
 	}
-	return nil
+	return
 }
 
 func (s *Session) Check(table Table, query string, values ...interface{}) (err error) {