fail on unusable native index without sources, create assets dir before marker delete

Launcher/WiiCompiled.Setup.Linux/DiscTool.cs

@@ -41,6 +41,7 @@ internal static class DiscTool
         // time, so it has to survive past this install (see Program.cs, which points dvd_root here).
         reporter.Progress(InstallStages.ExtractDisc, "Extracting the disc image", 4);
         var dataDir = Path.Combine(assetsDirectory, "DATA");
+        Directory.CreateDirectory(assetsDirectory);
         File.Delete(Path.Combine(assetsDirectory, ExtractionMarker));
         if (Directory.Exists(dataDir)) Directory.Delete(dataDir, recursive: true);
         await RunExtractAsync(nodTool, isoPath, dataDir, cancellationToken);

translator/src/Translator.Core/RuntimeNativeIndex.cs

@@ -87,6 +87,9 @@ public static class RuntimeNativeIndexBuilder
             : null;
         if (RuntimeNativeIndexFile.TryLoad(indexPath, expectedDigest, out var index, out var reason))
             return index;
+        if (expectedDigest is null)
+            throw new InvalidOperationException(
+                $"The native index {indexPath} is unusable ({reason}) and the native sources it describes are missing: {sourceRoot}");
 
         diagnostics?.WriteLine($"[translator] ignoring native index {indexPath} ({reason}); scanning {sourceRoot}");
         return Build(sourceRoot);

translator/tests/Translator.Tests/RuntimeNativeIndexTests.cs

@@ -149,6 +149,10 @@ public sealed class RuntimeNativeIndexTests
             AssertIndexEqual(built, RuntimeNativeIndexBuilder.Resolve(sources, indexPath, diagnostics));
             Assert.Equal("", diagnostics.ToString());
             Assert.Empty(RuntimeNativeIndexBuilder.Build(sources).Registrations);
+
+            File.WriteAllText(indexPath, "{}");
+            Assert.Throws<InvalidOperationException>(
+                () => RuntimeNativeIndexBuilder.Resolve(sources, indexPath, diagnostics));
         }
         finally
         {