@Provider
public class NotFoundExceptionMapper implements ExceptionMapper<org.jboss.resteasy.spi.NotFoundException>
{
    @Override
    Response toResponse(org.jboss.resteasy.spi.NotFoundException exception) {
         return Response.status(500).build();
         }
}

package demo.app;

import javax.ws.rs.NotFoundException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import javax.ws.rs.ext.ExceptionMapper;
import javax.ws.rs.ext.Provider;

@Provider
public class RestNotFoundExceptionHandler implements ExceptionMapper<NotFoundException> {
    @Override
    public Response toResponse(NotFoundException e) {
        return Response.ok().entity(e.toString() + "n").type(MediaType.TEXT_PLAIN).build();
    }
}

package demo.app;

import javax.servlet.http.HttpServletRequest;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.Context;
import javax.ws.rs.core.MediaType;

@Path("/")
public class AppRest {
    @GET
    @Path("millis")
    @Produces(MediaType.TEXT_PLAIN)
    public String refresh(@Context HttpServletRequest request) {
        try {
            return Long.toString(System.currentTimeMillis()) + "n";
        } catch (Exception e) {
            return e.toString() + "n";
        }
    }
}

package demo.app;

import javax.ws.rs.NotFoundException;
import org.jboss.resteasy.plugins.server.tjws.TJWSEmbeddedJaxrsServer;

public class AppMain {
    private static final int PORT = 8080;

    public static void main(String[] args) {
        AppRest appRest = new AppRest();
        TJWSEmbeddedJaxrsServer tjws = new TJWSEmbeddedJaxrsServer();
        tjws.setPort(PORT);
        tjws.setRootResourcePath("/rest");
        tjws.start();
        tjws.getDeployment().getRegistry().addSingletonResource(appRest);
        tjws.getDeployment().getProviderFactory().getExceptionMappers()
                .put(NotFoundException.class, new RestNotFoundExceptionHandler());
    }
}

jaxrs-api-3.0.9.Final.jar
resteasy-jaxrs-3.0.9.Final.jar
servlet-api-2.5.jar
tjws-3.0.9.Final.jar

$ curl http://127.0.0.1:8080/rest/millis
1419167594504

$ curl http://127.0.0.1:8080/rest/bad
javax.ws.rs.NotFoundException: Could not find resource for full path: http://127.0.0.1:8080/rest/bad