bug#19630: error message when referencing non-existing output not helpful

[Ludovic Courtès]

Commit f304c9c fixes that.  Now the result is something like this:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix build abiword -n
guix build: error: reference to invalid output 'binfds' of derivation 
'/gnu/store/7lxd5yri8s4vkailk1jg5bxiqrshblj1-glib-2.42.1.drv'
--8<---------------cut here---------------end--------------->8---

It doesn’t say the context in which that invalid reference occurred, but
it’s already much better.

Note that at the lowest level it’s possible to create a derivation that
refers to a non-existent output of some other derivation.  The problem
will go unnoticed until the ‘build-derivations’ RPC is called.

Thanks,
Ludo’.