Add a concept of experiment auto disable conditions

[brendanheywood]

Lets say we want to run an experiment which some potentially controversial code. We suspect it may cause an issue but we want to roll it out softly and then if there is an issue we can stop instantly without affecting more traffic

rough plan:

• [ ] configure an experiment with an error code(s) / string to watch for
• [ ] register set_error_handler after_config - might also need some extra core callbacks or something to detect them correctly
• [ ] if triggers log some details, time, stacktrace etc and disable the experiment