[SCB-2056]print a verbose log when client pool not initialized

foundations/foundation-vertx/src/main/java/org/apache/servicecomb/foundation/vertx/client/ClientPoolManager.java

@@ -105,6 +105,7 @@ protected CLIENT_POOL findByContext(Context targetContext) {
     // 2.vertx worker thread
     // 3.other vertx thread
     // select a existing context
+    assertPoolsInitialized();
     int idx = reactiveNextIndex.getAndIncrement() % pools.size();
     if (idx < 0) {
       idx = -idx;
@@ -113,7 +114,15 @@ protected CLIENT_POOL findByContext(Context targetContext) {
   }
 
   public CLIENT_POOL findThreadBindClientPool() {
+    assertPoolsInitialized();
     int idx = (int) (Thread.currentThread().getId() % pools.size());
     return pools.get(idx);
   }
+
+  private void assertPoolsInitialized() {
+    if (pools.size() == 0) {
+      throw new IllegalStateException("client pool not initialized successfully when making calls."
+          + "Please check if system boot up is ready or some errors happened when startup.");
+    }
+  }
 }