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/nodes.go

@@ -154,7 +154,7 @@ func buildCMDNode(ctx context.Context, rn *rawNode, toggles featuremgmt.FeatureT
 		return nil, fmt.Errorf("expression command type '%v' in expression '%v' not implemented", commandType, rn.RefID)
 	}
 	if err != nil {
-		return nil, fmt.Errorf("failed to parse expression '%v': %w", rn.RefID, err)
+		return nil, MakeParseError(rn.RefID, err)
 	}
 
 	return node, nil