bug#24361: guix graph backtrace on graphviz help message

[Ludovic Courtès]

Efraim Flashner <address@hidden> skribis:

> address@hidden:~$ time guix graph -t bag pbzip2 | dot -tpdf > pbzip2.pdf

[...]

> guix graph: error: scm_flush: Broken pipe

Hi Efraim!  It’s a feature, not a bug: ‘guix graph’ wasn’t able to write
everything to standard output, so it errors out.

There are other cases where it’s best to silently ignore that (‘guix
package -A’ does that for instance), but this is not one of them IMO.

Ludo’.