It would be great if Debug\ExceptionHandler had an option to output JSON. This is more relevant than ever with the new symfony/flex ideology IMO. Symfony is being used for API development more than ever, and having HTML output while running API spec testing, dredd, behat, etc. makes it really hard to see what the issue is.

Because this could happen before the kernel is booted, my suggestion is to have the option be done when calling Debug::enable() or new Kernel() or something early on if content-negotiation is not possible.