fix: support existing folders in single drive mode (#13254)

This PR however also proceeds to simplify the loading
of various subsystems such as

- globalNotificationSys
- globalTargetSys

converge them directly into single bucket metadata sys
loader, once that is loaded automatically every other
target should be loaded and configured properly.

fixes #13252

cmd/bucket-handlers.go

@@ -1380,6 +1380,12 @@ func (api objectAPIHandlers) PutBucketTaggingHandler(w http.ResponseWriter, r *h
 		return
 	}
 
+	// Check if bucket exists.
+	if _, err := objectAPI.GetBucketInfo(ctx, bucket); err != nil {
+		writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL)
+		return
+	}
+
 	if s3Error := checkRequestAuthType(ctx, r, policy.PutBucketTaggingAction, bucket, ""); s3Error != ErrNone {
 		writeErrorResponse(ctx, w, errorCodes.ToAPIErr(s3Error), r.URL)
 		return