download: retry with default prefix if not found


```
(Shiphero-API) tin@morocha:~/lab/Shiphero-API$ shbin dl compare_reqs.py
x content not found
(Shiphero-API) tin@morocha:~/lab/Shiphero-API$ shbin dl mgaitan/compare_reqs.py
✓ downloaded compare_reqs.py
```

I expected the first case to work