bug#18070: Hydra does not cope well with offline build machines

[Andreas Enge]

For instance here:
   http://hydra.gnu.org/build/79392

There are lines
lsh: Connect failed, (errno = 0)
guix offload: error: failed to register GC root for 
'/gnu/store/8nklnmzj317ahk6ynz7k5748lhma8cgn-GD_2_0_33.tar.xz.drv' on 
'#<<build-machine> name: "enge.fr" port: 2222 system: "mips64el-linux" user: 
"hydra" private-key: "/home/hydra/.lsh/identity" parallel-builds: 2 speed: 0.5 
features: ()>' (status: 256)
builder for `/gnu/store/8nklnmzj317ahk6ynz7k5748lhma8cgn-GD_2_0_33.tar.xz.drv' 
failed with exit code 1
@ hook-failed /gnu/store/8nklnmzj317ahk6ynz7k5748lhma8cgn-GD_2_0_33.tar.xz.drv 
- 256 builder for 
`/gnu/store/8nklnmzj317ahk6ynz7k5748lhma8cgn-GD_2_0_33.tar.xz.drv' failed with 
exit code 1
cannot build derivation 
`/gnu/store/i7xzs82ja6pwa641hdagq32z3m0g6c9y-gd-2.0.33.drv': 1 dependencies 
couldn't be built

It is normal that there is no connection to enge.fr:2222 because the machine
is offline. But hydra should notice the lack of response and build on another
machine instead.

Andreas