logs: use %o to format error

otherwise, they are printed as multi-line and this messes up tail+date formatting

src/scripts/backupupload.js

@@ -47,7 +47,7 @@ function throttledProgressCallback(msecs) {
     await settings.initCache();
 
     const [uploadError] = await safe(backuptask.upload(remotePath, format, dataLayoutString, throttledProgressCallback(5000)));
-    debug('upload completed. error: ', uploadError);
+    debug('upload completed. error: %o', uploadError);
 
     process.send({ result: uploadError ? uploadError.message : '' });