Improve FlushEntities() error logs (#5427)

* Improve FlushEntities() error logs

* log count before flush

Robust.Shared/GameObjects/EntityManager.cs

@@ -692,11 +692,23 @@ public bool Deleted([NotNullWhen(false)] EntityUid? uid)
         /// </summary>
         public virtual void FlushEntities()
         {
+            _sawmill.Info($"Flushing entities. Entity count: {Entities.Count}");
             BeforeEntityFlush?.Invoke();
             FlushEntitiesInternal();
 
             if (Entities.Count != 0)
-                _sawmill.Error("Failed to flush all entities");
+            {
+                _sawmill.Error($"Failed to flush all entities. Entity count: {Entities.Count}");
+                // Dump entity info, but avoid dumping ~50k errors if for whatever reason we failed to delete almost all entities.
+                // Using 512 as the limit, in case the problem entities are related to player counts on high-pop servers.
+                if (Entities.Count < 512)
+                {
+                    foreach (var uid in Entities)
+                    {
+                        _sawmill.Error($"Entity exists after flush: {ToPrettyString(uid)}");
+                    }
+                }
+            }
 
 #if EXCEPTION_TOLERANCE
             // Attempt to flush entities a second time, just in case something somehow caused an entity to be spawned
@@ -705,7 +717,7 @@ public virtual void FlushEntities()
 #endif
 
             if (Entities.Count != 0)
-                throw new Exception("Failed to flush all entities");
+                throw new Exception($"Failed to flush all entities. Entity count: {Entities.Count}");
 
             AfterEntityFlush?.Invoke();
         }