AST should not be serialized when there are compilation errors

[bamarsha]

AST serialization doesn't currently support lambda expression nodes, and the lambda lifting rewrite step isn't run when there are compilation errors. That means every compilation error in a project that contains a lambda expression will also have a serialization error, like that shown in #1387. While AST serialization should support lambdas, there's also not really a reason for it to run when there are compilation issues.