Fix uploader API to handle write stream errors

When the upload is aborted/abandoed because the source file is missing,
the write stream is never destroyed. This dangling write stream can
later error and cause a crash.

Instead, create the write stream on demand and nearer to pipeline() to
make sure we do 'error' handling.

src/backupformat/tgz.js

@@ -146,16 +146,17 @@ async function tarPack(dataLayout, encryption, uploader, progressCallback) {
         progressCallback({ message: `Uploading backup ${transferred}M@${speed}MBps` });
     });
 
+    // careful not to have async code between here and pipeline() for 'error' handling
     const pack = tar.pack();
-
     const hash = new HashStream();
+    const destStream = uploader.createStream();
 
     let pipeline = null;
     if (encryption) {
         const encryptStream = new EncryptStream(encryption);
-        pipeline = safe(stream.pipeline(pack, gzip, encryptStream, ps, hash, uploader.stream));
+        pipeline = safe(stream.pipeline(pack, gzip, encryptStream, ps, hash, destStream));
     } else {
-        pipeline = safe(stream.pipeline(pack, gzip, ps, hash, uploader.stream));
+        pipeline = safe(stream.pipeline(pack, gzip, ps, hash, destStream));
     }
 
     let fileCount = 0;