Fix the code lost with multi errors (#22877)

Signed-off-by: Nyah01 <yang.cen@zilliz.com>

internal/util/merr/errors.go

@@ -166,6 +166,12 @@ func (e multiErrors) Unwrap() error {
 	if len(e.errs) <= 1 {
 		return nil
 	}
+	// To make merr work for multi errors,
+	// we need cause of multi errors, which defined as the last error
+	if len(e.errs) == 2 {
+		return e.errs[1]
+	}
+
 	return multiErrors{
 		errs: e.errs[1:],
 	}

internal/util/merr/errors_test.go

@@ -138,6 +138,11 @@ func (s *ErrSuite) TestCombineOnlyNil() {
 	s.Nil(err)
 }
 
+func (s *ErrSuite) TestCombineCode() {
+	err := Combine(WrapErrIoFailed("test"), WrapErrCollectionNotFound(1))
+	s.Equal(Code(ErrCollectionNotFound), Code(err))
+}
+
 func TestErrors(t *testing.T) {
 	suite.Run(t, new(ErrSuite))
 }