SSE: Return error messages instead of 500 on SSE command parse errors (#109480)

fixes #108897

---------

Co-authored-by: Kyle Brandt <kyle@grafana.com>

pkg/expr/service_test.go

@@ -148,6 +148,25 @@ func TestDSQueryError(t *testing.T) {
 	require.Equal(t, fp(42), res.Responses["C"].Frames[0].Fields[0].At(0))
 }
 
+func TestParseError(t *testing.T) {
+	resp := map[string]backend.DataResponse{}
+
+	queries := []Query{
+		{
+			RefID:      "A",
+			DataSource: dataSourceModel(),
+			JSON:       json.RawMessage(`{ "datasource": { "uid": "__expr__", "type": "__expr__"}, "type": "math", "expression": "asdf" }`),
+		},
+	}
+
+	s, req := newMockQueryService(resp, queries)
+
+	_, err := s.BuildPipeline(t.Context(), req)
+	require.ErrorContains(t, err, "parse")
+	require.ErrorContains(t, err, "math")
+	require.ErrorContains(t, err, "asdf")
+}
+
 func TestSQLExpressionCellLimitFromConfig(t *testing.T) {
 	tests := []struct {
 		name            string