Fail when user wants to provided metadata for already existing library version

Author: dnestoro

tests/tck-build-logic/src/main/java/org/graalvm/internal/tck/ScaffoldTask.java

@@ -252,7 +252,7 @@ private void writeCoordinatesMetadataRootJson(Path metadataRoot, Coordinates coo
 
     private void updateCoordinatesMetadataRootJson(Path metadataRoot, Coordinates coordinates) throws IOException {
         if (!shouldAddNewMetadataEntry(metadataRoot, coordinates)) {
-            return;
+            throw new RuntimeException("Metadata for " + coordinates + " already exists!");
         }
 
         File metadataIndex = metadataRoot.resolve("index.json").toFile();