Which model class does spring use for error responses of Request Mapping methods?

I am designing a POJO for custom error scenarios of my @RequestMapping methods.

Spring by default throws a HttpMessageNotReadableException if a POST is made without any body and returns the error response very nicely as something like this:

{
    "timestamp": "2017-07-28T18:54:11.867+0000",
    "status": 400,
    "error": "Bad Request",
    "exception": "org.springframework.http.converter.HttpMessageNotReadableException",
    "message": "Required request body is missing: public org.springframework.http.ResponseEntity<some.package.name.SomeResponseClass> some.package.name.SomeControllerClass.someRequestMappingMethod(java.util.Map<java.lang.String, java.lang.String>, some.package.name.SomeRequestClass)",
    "path": "/somepath/"
}

Similarly if validation for any field annotated with @Valid fails, it throws MethodArgumentNotValidException and returns the error response as:

{
    "timestamp": "2017-07-28T17:23:53.102+0000",
    "status": 400,
    "error": "Bad Request",
    "exception": "org.springframework.web.bind.MethodArgumentNotValidException",
    "errors": [
        {
            "codes": [
                "NotNull.incident.someField",
                "NotNull.someField",
                "NotNull.java.lang.String",
                "NotNull"
            ],
            "arguments": [
                {
                    "codes": [
                        "someObject.someField",
                        "someField"
                    ],
                    "arguments": null,
                    "defaultMessage": "someField",
                    "code": "someField"
                }
            ],
            "defaultMessage": "someField is null",
            "objectName": "someObject",
            "field": "someField",
            "rejectedValue": null,
            "bindingFailure": false,
            "code": "NotNull"
        }
    ],
    "message": "Validation failed for object='someObject'. Error count: 2",
    "path": "/somepath/"
}

I an having an understanding that in either of the above case Spring must be returning a ResponseEntity object with headers and body wrapped in it. And there must be a model class representing the body having fields like above i.e. timestamp, status, error etc. I just wished to visit that class to get an idea of what all fields Spring has included in their error response model. Or may be I might want to extend it. I have spent quite a huge amount of time by now searching for that class, but couldn't find.

The best I could find is ResponseEntityExceptionHandler#handleException(), but my debugger will not even go at the break-point set there!!

Can anyone point me to the package where it exists?