Failures without internet connection

[Andreas Enge <andreas@enge.fr>]

Download of substituted binaries fails when one is offline. This is 
logical, but should guix then not gracefully fall back to compilation if 
the sources have already been downloaded?

In any case, catching the error and emitting an error message would 
probably be preferable to

$ guix build hello
The following file will be downloaded:
   /nix/store/7g06kqdm6qjbzdppnryyvjl2bz82r8dh-hello-2.8
@ substituter-started /nix/store/7g06kqdm6qjbzdppnryyvjl2bz82r8dh-hello-2.8 
/usr/local/guix-git/libexec/guix/substitute-binary
Backtrace:
In ice-9/boot-9.scm:
 149: 12 [catch #t #<catch-closure 1c131a0> ...]
 157: 11 [#<procedure 1bb10f0 ()>]
In unknown file:
   ?: 10 [catch-closure]
In ice-9/boot-9.scm:
  63: 9 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 407: 8 [eval # #]
In ice-9/boot-9.scm:
2111: 7 [save-module-excursion #<procedure 1bb6100 at 
ice-9/boot-9.scm:3646:3 ()>]
3651: 6 [#<procedure 1bb6100 at ice-9/boot-9.scm:3646:3 ()>]
In unknown file:
   ?: 5 [load-compiled/vm "/root/.cache/guile/ccache/2.0-
LE-8-2.0/usr/local/guix-git/bin/guix.go"]
In guix/ui.scm:
 460: 4 [run-guix-command substitute-binary "--substitute" ...]
In guix/scripts/substitute-binary.scm:
 428: 3 [guix-substitute-binary "--substitute" ...]
In guix/web.scm:
 151: 2 [loop #]
In web/client.scm:
  45: 1 [open-socket-for-uri #]
In unknown file:
   ?: 0 [getaddrinfo "hydra.gnu.org" "http" ...]

ERROR: In procedure getaddrinfo:
ERROR: In procedure getaddrinfo: Der Name oder der Dienst ist nicht bekannt
@ substituter-failed /nix/store/7g06kqdm6qjbzdppnryyvjl2bz82r8dh-hello-2.8 
256 fetching path `/nix/store/7g06kqdm6qjbzdppnryyvjl2bz82r8dh-hello-2.8' 
failed with exit code 1
guix build: error: build failed: some substitutes for the outputs of 
derivation `/nix/store/bc6hgclkr7a3yq3xh7h6m7myagfdwvdj-hello-2.8.drv' 
failed; try `--fallback'


In the past, I think I also ran into a similar problem even when using "--
no-substitutes", as guix tried in vain to check for newer versions of gnu 
packages, but I cannot reproduce this problem now.

Andreas