Fix AssetContainer behavior with immediate load errors

The AssetContainer was getting "stuck" in the case that it tried to load a missing asset that was already registered with the AssetManager as missing.  This fixes the bug, as well as adding a unit test for the specific condition.

Code/Framework/AzCore/AzCore/Asset/AssetContainer.cpp

@@ -269,13 +269,13 @@ namespace AZ
             {
                 for (auto& [assetId, dependentAsset] : m_dependencies)
                 {
-                    if (dependentAsset->IsReady())
+                    if (dependentAsset->IsReady() || dependentAsset->IsError())
                     {
                         HandleReadyAsset(dependentAsset);
                     }
                 }
             }
-            if (auto asset = m_rootAsset.GetStrongReference(); asset.IsReady())
+            if (auto asset = m_rootAsset.GetStrongReference(); asset.IsReady() || asset.IsError())
             {
                 HandleReadyAsset(asset);
             }
@@ -496,10 +496,10 @@ namespace AZ
                         m_waitingCount -= 1;
                         disconnectEbus = true;
 
-                        if (m_waitingAssets.empty())
-                        {
-                            allReady = true;
-                        }
+                    }
+                    if (m_waitingAssets.empty())
+                    {
+                        allReady = true;
                     }
                 }
 
@@ -510,8 +510,15 @@ namespace AZ
                 }
             }
 
-            if (allReady && m_initComplete)
+            // If there are no assets left to be loaded, trigger the final AssetContainer notification (ready or canceled).
+            // We guard against prematurely sending it (m_initComplete) because it's possible for assets to get removed from our waiting
+            // list *while* we're still building up the list, so the list would appear to be empty too soon.
+            // We also guard against sending it multiple times (m_finalNotificationSent), because in some error conditions, it may be
+            // possible to try to remove the same asset multiple times, which if it's the last asset, it could trigger multiple
+            // notifications.
+            if (allReady && m_initComplete && !m_finalNotificationSent)
             {
+                m_finalNotificationSent = true;
                 if (m_rootAsset)
                 {
                     AssetManagerBus::Broadcast(&AssetManagerBus::Events::OnAssetContainerReady, this);

Code/Framework/AzCore/AzCore/Asset/AssetContainer.h

@@ -137,6 +137,7 @@ namespace AZ
             AZStd::atomic_int m_invalidDependencies{ 0 };
             AZStd::unordered_set<AZ::Data::AssetId> m_unloadedDependencies;
             AZStd::atomic_bool m_initComplete{ false };
+            AZStd::atomic_bool m_finalNotificationSent{false};
 
             mutable AZStd::recursive_mutex m_preloadMutex;
             // AssetId -> List of assets it is still waiting on