@Test
    public void testCustomErrorMessage() throws Exception {
        String errorMessage = "testCustomErrorMessage";
        verifyErrorMessage(produceResponse(tp0, 0L, Errors.INVALID_REQUEST, 0, -1, errorMessage), errorMessage);
    }
Tests the code path where the target node to send FindCoordinator or InitProducerId
is not ready.
clients/src/test/java/org/apache/kafka/clients/producer/internals/SenderTest.java