Addressing PR comments

Author: LikeTheSalad

disk-buffering/src/main/java/io/opentelemetry/contrib/disk/buffering/internal/storage/FileSignalStorage.java

@@ -49,7 +49,8 @@ public CompletableFuture<WriteResult> write(Collection<T> items) {
         return CompletableFuture.completedFuture(WriteResult.successful());
       }
       logger.fine("Could not store batch in disk.");
-      return CompletableFuture.completedFuture(WriteResult.error(null));
+      return CompletableFuture.completedFuture(
+          WriteResult.error(new Exception("Could not store batch in disk for an unknown reason.")));
     } catch (IOException e) {
       logger.log(
           Level.WARNING,

disk-buffering/src/main/java/io/opentelemetry/contrib/disk/buffering/internal/storage/FolderManager.java

@@ -31,7 +31,8 @@ public static FolderManager create(
       File destinationDir, FileStorageConfiguration configuration, Clock clock) {
     if (destinationDir.isFile()) {
       throw new IllegalArgumentException("destinationDir must be a directory");
-    } else if (!destinationDir.exists()) {
+    }
+    if (!destinationDir.exists()) {
       if (!destinationDir.mkdirs()) {
         throw new IllegalStateException("Could not create dir: " + destinationDir);
       }