Fix response tag handling

Fixes https://linear.app/serverless/issue/SC-1653/python-sdk-error-user-instrumented-functions-and-all-functions-started
My only explanation is that somehow the customer is using AWS Lambda to implement an http API but failing to return a dictionary with "statusCode" field in it and we fail to properly surface the issue.
This will fix the status code extraction logic to make sure it doesn't fail and instead just report that the status code is missing.

Unit tested

serverless / dashboard