Fix upgrade prompt

When running the upgrade command, the prompt `(Y|n)` indicates that yes is the default answer when pressing return. When return is pressed, the following error message appears

```bash
% trousseau upgrade
You are about to upgrade trousseau data store trousseau.asc (version 0.3.0) up to version 0.3.4. Proceed? [Y/n] 
Error: unexpected newline
```

This fix defaults correctly to "y" if nothing was scanned by checking the `count` return variable instead of `error`.