improve thread pool rejection message

src/main/java/org/elasticsearch/common/util/concurrent/EsAbortPolicy.java

@@ -49,7 +49,16 @@ public class EsAbortPolicy implements XRejectedExecutionHandler {
             }
         }
         rejected.inc();
-        throw new EsRejectedExecutionException("rejected execution of [" + r.getClass().getName() + "]");
+        StringBuilder sb = new StringBuilder("rejected execution ");
+        if (executor.isShutdown()) {
+            sb.append("(shutting down) ");
+        } else {
+            if (executor.getQueue() instanceof SizeBlockingQueue) {
+                sb.append("(queue capacity ").append(((SizeBlockingQueue) executor.getQueue()).capacity()).append(") ");
+            }
+        }
+        sb.append("on ").append(r.toString());
+        throw new EsRejectedExecutionException(sb.toString());
     }
 
     @Override

src/main/java/org/elasticsearch/common/util/concurrent/SizeBlockingQueue.java

@@ -51,6 +51,10 @@ public class SizeBlockingQueue<E> extends AbstractQueue<E> implements BlockingQu
         return size.get();
     }
 
+    public int capacity() {
+        return this.capacity;
+    }
+
     @Override
     public Iterator<E> iterator() {
         final Iterator<E> it = queue.iterator();