improve error message for `guix environment'

[Tomáš Čech]

When network issue occurs during `guix environment <package>' call,
the error message is too cryptic:


$ guix environment gnash
;;; note: source file /Devel/git/guix/gnu/packages/dictionaries.scm
;;;       newer than compiled /Devel/git/guix/gnu/packages/dictionaries.go
;;; note: source file /Devel/git/guix/gnu/packages/graphics.scm
;;;       newer than compiled /Devel/git/guix/gnu/packages/graphics.go
substitute-binary: guix substitute-binary: warning: failed to look up host 
'hydra.gnu.org' (Temporary failure in name resolution), substituter disabled
The following derivations will be built:
  /gnu/store/9z9bpm9pwhd3dwx5kfwwqbirfd3v6bn5-coreutils-8.23.tar.xz.drv
  /gnu/store/y1b1dvljiyz4s8r2bhp3ch9nijm8dnh6-coreutils-8.23.tar.xz.drv
  /gnu/store/0p2nh8j0ad1qsd2ipwhlp6zais1jbc3m-glib-2.42.1.tar.xz.drv
  /gnu/store/wmy3xalfpg9vw4awfflgl26mynxv7554-glib-2.42.1.tar.xz.drv
  /gnu/store/pph3srjx594rcq0cjah679rd5xc7nqi2-coreutils-8.23.drv
  /gnu/store/k4y35xga43ib54688i76n11phz1xhrnz-giflib-4.2.3.tar.bz2.drv
starting download of 
`/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2' from 
`http://prdownloads.sourceforge.net/giflib/giflib-4.x/giflib-4.2.3.tar.bz2'..
.
ERROR: In procedure getaddrinfo: Temporary failure in name resolution
starting download of 
`/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2' from 
`http://heanet.dl.sourceforge.net/sourceforge/giflib/giflib-4.x/giflib-4.2.3.
tar.bz2'...
ERROR: In procedure getaddrinfo: Temporary failure in name resolution
starting download of 
`/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2' from 
`http://surfnet.dl.sourceforge.net/sourceforge/giflib/giflib-4.x/giflib-4.2.3
.tar.bz2'...
ERROR: In procedure getaddrinfo: Temporary failure in name resolution
starting download of 
`/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2' from 
`http://dfn.dl.sourceforge.net/sourceforge/giflib/giflib-4.x/giflib-4.2.3.tar
.bz2'...
ERROR: In procedure getaddrinfo: Temporary failure in name resolution
starting download of 
`/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2' from 
`http://mesh.dl.sourceforge.net/sourceforge/giflib/giflib-4.x/giflib-4.2.3.ta
r.bz2'...
ERROR: In procedure getaddrinfo: Temporary failure in name resolution
starting download of 
`/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2' from 
`http://ovh.dl.sourceforge.net/sourceforge/giflib/giflib-4.x/giflib-4.2.3.tar
.bz2'...
ERROR: In procedure getaddrinfo: Temporary failure in name resolution
starting download of 
`/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2' from 
`http://osdn.dl.sourceforge.net/sourceforge/giflib/giflib-4.x/giflib-4.2.3.ta
r.bz2'...
ERROR: In procedure getaddrinfo: Temporary failure in name resolution
failed to download "/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3.tar.bz2" 
from "mirror://sourceforge/giflib/giflib-4.x/giflib-4.2.3.tar.bz2"
builder for 
`/gnu/store/k4y35xga43ib54688i76n11phz1xhrnz-giflib-4.2.3.tar.bz2.drv' failed 
to produce output path `/gnu/store/xc4r4ym38y83i4ncdavrcg8h8m4vm499-giflib-4.2.3
.tar.bz2'
cannot build derivation 
`/gnu/store/z8f07h9gix3p33jqv6sckqd2ynn40mv5-giflib-4.2.3.drv': 1 dependencies 
couldn't be built
Backtrace:
In ice-9/boot-9.scm:
157: 14 [catch #t #<catch-closure 12fd7e0> ...]
In unknown file:
  ?: 13 [apply-smob/1 #<catch-closure 12fd7e0>]
In ice-9/boot-9.scm:
 63: 12 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
432: 11 [eval # #]
In ice-9/boot-9.scm:
2401: 10 [save-module-excursion #<procedure 131a940 at ice-9/boot-9.scm:4045:3 
()>]
4050: 9 [#<procedure 131a940 at ice-9/boot-9.scm:4045:3 ()>]
1724: 8 [%start-stack load-stack ...]
1729: 7 [#<procedure 1332ea0 ()>]
In unknown file:
  ?: 6 [primitive-load "/Devel/git/guix/scripts/guix"]
In guix/ui.scm:
849: 5 [run-guix-command environment "gnash"]
In guix/scripts/environment.scm:
224: 4 [guix-environment "gnash"]
In guix/store.scm:
936: 3 [run-with-store # ...]
In guix/scripts/environment.scm:
213: 2 [#<procedure 3d23e70 at guix/scripts/environment.scm:213:12 (state)> #]
In guix/store.scm:
883: 1 [#<procedure 3d23c60 at guix/store.scm:882:28 (store)> #]
580: 0 [build-things # #]

guix/store.scm:580:0: In procedure build-things:
guix/store.scm:580:0: Throw to key `srfi-34' with args `(#<condition 
&nix-protocol-error [message: "build of 
`/gnu/store/z8f07h9gix3p33jqv6sckqd2ynn40mv5-giflib-4.2.3.drv
' failed" status: 1] 3d64ba0>)'.