Show alert if .NET assemblies dir does not exist

[raulsntos]

Shows a clearer message to users when the data directory is missing.

• Resolves this comment: C#: Add platform name to the exported data directory #78846 (comment).

[akien-mga]

Looks great!

[akien-mga]

Thanks!

[RedworkDE]

AFAIK previously you could run a non-C# project with the dotnet export template and it would work just fine except that it prints that error in the console, while now it will show a message box.
Should this error be conditional to some indicator in the pck that the data dir even should exist or is this case rare enough to not matter?

[akien-mga]

That's a good point. Ideally it should be possible to export a non-C# project and have it just work. Maybe this could be dependent on the presence of the res://.godot/mono/ folder, or some other metadata that gets set on export?

[raulsntos]

We can probably extract the GDMono::should_initialize method from #74089.

• Edit: Implemented by C#: Allow exporting games without C# #83422.