@Path("test")
public interface IService {
    @POST
    @Path("test")
    public void test() throws Exception;
}

public class Service implements IService {
    @Override
    public void test() throws Exception {
        throw new Exception("Blah");
    }
}

@Provider
public class ServerExceptionHandler implements ExceptionMapper<Exception> {
    public Response toResponse(Exception exception) {
        return Response.serverError().entity("Blah blah blah").build();
    }
}

IService service = ProxyFactory.create(IService.class, "http://localhost:8080/resteasy-test/test");
try {
    service.test();
} catch (ClientResponseFailure e) {
    Response resp = e.getResponse();
    System.out.println(((ClientResponse) resp).getEntity(String.class));
}

ResteasyClient client = new ResteasyClientBuilder().build();
ResteasyWebTarget target = (ResteasyWebTarget) client.target("http://localhost:8080/resteasy-test/test");
IService service = target.proxy(IService.class);
try {
    service.test();
} catch (InternalServerErrorException e) {
    Response resp = e.getResponse();
    System.out.println(resp.readEntity(String.class));
}

Exception in thread "main" java.lang.IllegalStateException: Response is closed.
	at org.jboss.resteasy.specimpl.BuiltResponse.abortIfClosed(BuiltResponse.java:254)
	at org.jboss.resteasy.client.jaxrs.internal.ClientResponse.readEntity(ClientResponse.java:148)
	at org.jboss.resteasy.specimpl.BuiltResponse.readEntity(BuiltResponse.java:217)