Event handlers are called while trying to load bad save file

[Player701]

The title of this thread is yet another unfortunate victim of the character limit.
The proper title should be: "GZDoom calls event handlers while trying to load a save file with unknown classes".

While attempting to load a save file that has references to unknown class types, GZDoom will try to call local event handlers that are present in the save file. This can lead to unexpected ZScript errors popping up because these handlers don't seem to be initialized properly.

Steps to reproduce:

1. Download test1.wad and run it with Doom II MAP01.
2. Save the game and quit.
3. Download test2.wad and run GZDoom with it.
4. Load the save file that was made on step 2. GZDoom will print some messages to the console, and the last line will be "OnRegister". This message is produced by the TestEventHandler that is present in the save file. Note that GZDoom will also crash, though it is probably due to a different issue.

Tested with GZDoom g4.5pre-140-ge74b9f195

[Graf Zahl]

It now performs a proper abort of the loading process when the object deserializer fails.