@Override
    public Mono<User> getUser(String username) {
        return client.get(User.class, username)
            .onErrorMap(ExtensionNotFoundException.class, e -> new UserNotFoundException(username));
    }
@Test
    void shouldThrowExceptionIfUserNotFoundInExtension() {
        when(client.get(eq(User.class), eq("faker"))).thenReturn(
            Mono.error(new ExtensionNotFoundException(fromExtension(User.class), "faker")));
        StepVerifier.create(userService.getUser("faker"))
            .verifyError(UserNotFoundException.class);

        verify(client, times(1)).get(eq(User.class), eq("faker"));
    }