[FEATURE] Allow saves to load with parts missing

[jan-bures]

Feature Information

Feature description

This issue can be divided into two separate but very related scenarios:

1. There's an in-flight vessel with a specific part "A". Let's say that part A comes from a mod, and the user disables or uninstalls the mod in between two play sessions. When they come back to the campaign that contains the loaded vessel with the part, the game will freeze during loading at the "Creating Vessel" loading action:
   
   The following error causes this freeze:
   

2. There are no vessels with missing parts in flight, but the user has such a vessel saved in VAB. When they attempt to load the vessel into the editor, they don't see any message or error, there's only an exception in a log, which is not very user friendly:
   

Solution proposal

1. Implement some sort of a system similar to KSP 1, where we would check during the save loading whether any of the vessels in the save have missing parts, and if so, don't load those vessels and warn the user:
   
   

2. This should be relatively easy - just patch the ObjectAssemblyBuilderFileIO.LoadWorkspaceFromFile method and display an error dialog when the vessel cannot be loaded. Example from KSP 1:
   
   

[cheese3660]

Hmm, the best way I can think to do this is to completely replace the flow actions code with a harmony patch

[cheese3660]

But to give the actual part errors would require a deeper patch to know what parts weren't loaded