slapos/cli: Nicer errors for cachelookup binary-sr
Change the error message when a software release is not found in the binary cache, from:
Error while looking object https://lab.nexedi.com/nexedi/slapos/raw/1.0.372/software/wendelin/software.cfg
Traceback (most recent call last):
<clipped>
urllib.error.HTTPError: HTTP Error 404: Not Found
to:
Software release not found in binary cache: <software_url>