Better error messages

[hololeap]

I tired running random-build and got

Could not get list of all available packages

with no indication of why or what the underlying error was.

I dug through the source code and pieced together that it was running

pquery --no-version --repo haskell

and that it was failing because of a misconfiguration on my sandbox

pquery: error: repos.conf: default repo 'gentoo' is undefined or invalid

---

It should generate more useful errors that include the command that was run, the exit code, and the captured stdout and stderr buffers.