Version 1.5.68

Improve the error messages returned when a request body or a filter is not valid JSON. The 422 message now names the problem, shows where it is and the text around it, and adds a hint with common causes. A request body that is not UTF-8 text now gets a 422 with a clear message instead of a server error.
Add response headers X-RateLimit-Limit, X-RateLimit-Remaining, and X-RateLimit-Reset on rate-limited endpoints to help integrators stay within the per-second limit.