register_shutdown_function in HTTPBulkToolsResponse causes invalid JSON

[kinglozzer]

In the event of a PHP warning being emitted, error_get_last() returns an error, which means that an extra message/status code is output after the JSON object in the response body - resulting in invalid JSON.

https://github.com/colymba/GridFieldBulkEditingTools/blob/master/src/BulkTools/HTTPBulkToolsResponse.php#L366-L378

I would suggest removing this behaviour entirely, or failing that at least check the type of error is fatal before outputting it

Oh, and exit() causes other registered shutdown functions to not fire, so my Sentry error handler never picked up the actual cause of the warning 😅