Using inferiors

[Philip McGrath]

I have what I think would be a simple use of inferiors, but it is failing to 
build. I'd like to understand both (1) why and (2) how to get a more 
informative error message.

```
philip@bastet:/tmp$ cat demo.scm 
(use-modules
 (guix channels)
 (guix inferior)
 (ice-9 match))

(define glibc-2.23-channels
  (list (channel
         (name 'guix)
         (url "https://git.savannah.gnu.org/git/guix.git";)
         (commit
          ;; glibc-2.23 was removed in commit:
          ;;    ebd1ba713cbefc9ad5dac609255e1344a328e360
          ;; This commit is the parent of that one:
          "ccc1d743a64fd71bee1a27f1f495978989b41126")
         (introduction
          (make-channel-introduction
           "9edb3f66fd807b096b48283debdcddccfea34bad"
           (openpgp-fingerprint
            "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"))))))

(define inferior
  (inferior-for-channels glibc-2.23-channels))

(match (lookup-inferior-packages inferior "glibc" "2.23")
  ((pkg)
   pkg))
philip@bastet:/tmp$ guix build -f demo.scm 
guix build: error: build of 
`/gnu/store/pmkm9g0w14zymdpzjh8db5c7smv7ia4j-glibc-2.23.drv' failed
```