Increase robustneess of iHat url bootsrapping

- Initialize config.ihat_supported_formats regardless of request result
- When request is not JSON, reject it and log a warning
- Only assign ihat_supported_formats if it can be parsed as json
I think that the context is correct, given that we're testing that any HTML response (it is a variant of an invalid response, that is not JSON). Let me know if you think I'm misunderstanding your comment.