JsonProcessingExceptionMapper should return a 400 response rather than a 500

The JsonProcessingExceptionMapper was introduced in RESTEASY-3380. The response if a parsing error occurs returns a 500 (INTERNAL SERVER ERROR). However, arguably this should be a 400 (BAD REQUEST). This could be considered backwards incompatible. However, it's definitely the appropriate response. Previously in some cases it would have been 400 anyway given the deleted UnrecognizedPropertyExceptionHandler.

In other words, we somewhat broke compatibility as it is and we should make it right.