How to write a proper global error handler with Spring MVC / Spring Boot

Have a look at ControllerAdvice
You could do something like this:

@ControllerAdvice
public class ExceptionHandlerController {

    public static final String DEFAULT_ERROR_VIEW = "error";

    @ExceptionHandler(value = {Exception.class, RuntimeException.class})
    public ModelAndView defaultErrorHandler(HttpServletRequest request, Exception e) {
            ModelAndView mav = new ModelAndView(DEFAULT_ERROR_VIEW);

        mav.addObject("datetime", new Date());
        mav.addObject("exception", e);
        mav.addObject("url", request.getRequestURL());
        return mav;
    }
}