Try improve PVS exception tolerance a bit more

[ElectroJr]

Instead of using the normal EntityTerminatingEvent system subscription, PVS system now uses a separate C# event that gets invoked before the normal system event. This might prevent some bugs caused by content code throwing exceptions while handling the event?

I don't know if thats what caused this, but smug just keeps discovering cursed PVS errors:

 at Robust.Shared.Utility.DebugTools.AssertNull(Object arg, String message) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/Utility/DebugTools.cs:line 250
   at Robust.Server.GameStates.PvsSystem.UpdatePosition(EntityUid uid, TransformComponent xform, MetaDataComponent meta, EntityUid oldParent) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Server/GameStates/PvsSystem.Entity.cs:line 38
   at Robust.Server.GameStates.PvsSystem.OnEntityMove(MoveEvent& ev) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Server/GameStates/PvsSystem.Entity.cs:line 14
   at Robust.Shared.GameObjects.SharedTransformSystem.RaiseMoveEvent(Entity`2 ent, EntityUid oldParent, Vector2 oldPosition, Angle oldRotation, Nullable`1 oldMap) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/GameObjects/Systems/SharedTransformSystem.cs:line 283
   at Robust.Shared.GameObjects.SharedTransformSystem.SetCoordinates(Entity`2 entity, EntityCoordinates value, Nullable`1 rotation, Boolean unanchor, TransformComponent newParent, TransformComponent oldParent) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/GameObjects/Systems/SharedTransformSystem.Component.cs:line 585
   at Robust.Shared.GameObjects.SharedTransformSystem.DetachEntityInternal(EntityUid uid, TransformComponent xform, MetaDataComponent meta, TransformComponent oldXform, Boolean terminating) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/GameObjects/Systems/SharedTransformSystem.Component.cs:line 1470
   at Robust.Shared.GameObjects.SharedTransformSystem.DetachEntity(EntityUid uid, TransformComponent xform, MetaDataComponent meta, TransformComponent oldXform, Boolean terminating) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/GameObjects/Systems/SharedTransformSystem.Component.cs:line 1399
   at Robust.Shared.GameObjects.EntityManager.RecursiveDeleteEntity(EntityUid uid, MetaDataComponent metadata, TransformComponent transform, TransformComponent parentXform) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/GameObjects/EntityManager.cs:line 581
   at Robust.Shared.GameObjects.EntityManager.DeleteEntity(EntityUid e, MetaDataComponent meta, TransformComponent xform) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/GameObjects/EntityManager.cs:line 535
   at Robust.Shared.GameObjects.EntityManager.DeleteEntity(Nullable`1 uid) in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.Shared/GameObjects/EntityManager.cs:line 502
   at Content.IntegrationTests.Tests.EntityTest.<>c__DisplayClass5_0.<AllComponentsOneToOneDeleteTest>b__1() in /home/runner/work/RMC-14/RMC-14/Content.IntegrationTests/Tests/EntityTest.cs:line 409
   at NUnit.Framework.Assert.Multiple(TestDelegate testDelegate)
   at Content.IntegrationTests.Tests.EntityTest.<>c__DisplayClass5_0.<AllComponentsOneToOneDeleteTest>b__0() in /home/runner/work/RMC-14/RMC-14/Content.IntegrationTests/Tests/EntityTest.cs:line 375
   at Robust.UnitTesting.RobustIntegrationTest.IntegrationGameLoop.SingleThreadRunUntilEmpty() in /home/runner/work/RMC-14/RMC-14/RobustToolbox/Robust.UnitTesting/RobustIntegrationTest.cs:line 1097