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/storage/interface.js

@@ -67,7 +67,7 @@ async function upload(config, limits, backupFilePath) {
     assert.strictEqual(typeof limits, 'object');
     assert.strictEqual(typeof backupFilePath, 'string');
 
-    // Result: { stream, finish() callback }
+    // Result: { createStream(), finish() callback }
     throw new BoxError(BoxError.NOT_IMPLEMENTED, 'upload is not implemented');
 }