cells: update log message to describe why enqueing a message failed.

The current log message does not describe why a message was thrown
away.  We have observed situations where the actual cause is not
obvious.

With this patch, the cause is logged.  The patch also distinguishes
between reply and non-reply messages.

Target: master
Requires-notes: no
Requires-book: no
Patch: https://rb.dcache.org/r/8225/
Acked-by: Tigran Mkrtchyan

modules/cells/src/main/java/dmg/cells/nucleus/CellNucleus.java

@@ -841,7 +841,7 @@ void addToEventQueue(MessageEvent ce)
                 synchronized (_waitHash) {
                     _waitHash.put(msg.getLastUOID(), lock);
                 }
-                LOGGER.error("Message queue overflow. Dropping {}", ce);
+                LOGGER.error("Dropping reply: {}", e.getMessage());
             }
         } else {
             try {
@@ -851,7 +851,7 @@ void addToEventQueue(MessageEvent ce)
             } catch (RejectedExecutionException e) {
                 EventLogger.queueEnd(ce);
                 _eventQueueSize.decrementAndGet();
-                LOGGER.error("Message queue overflow. Dropping {}", ce);
+                LOGGER.error("Dropping message: {}", e.getMessage());
             }
         }
     }
@@ -1069,6 +1069,12 @@ public void run()
                 }
             }
         }
+
+        @Override
+        public String toString()
+        {
+            return "Delivery-of-" + _message;
+        }
     }
 
     private class DeliverMessageTask implements Runnable
@@ -1111,5 +1117,11 @@ public void run()
                 }
             }
         }
+
+        @Override
+        public String toString()
+        {
+            return "Delivery-of-" + _event;
+        }
     }
 }