guix weather exit status?

[Leo Famulari]

When a substitute is not available for all specified substitute servers,
`guix weather` exits with a return code of '1', signaling failure.

For example:

------
$ ./pre-inst-env guix weather linux-libre; echo $?
computing 1 package derivations for x86_64-linux...
looking for 1 store items on http://ci.guix.gnu.org...
http://ci.guix.gnu.org
  100.0% substitutes available (1 out of 1)
  at least 234.8 MiB of nars (compressed)
  86.3 MiB on disk (uncompressed)

  38 queued builds
      aarch64-linux: 31 (81.6%)
      powerpc64le-linux: 1 (2.6%)
      armhf-linux: 1 (2.6%)
      i686-linux: 5 (13.2%)
  build rate: .00 builds per hour
      x86_64-linux: 0.00 builds per hour
      aarch64-linux: 0.00 builds per hour
      i686-linux: 0.00 builds per hour
looking for 1 store items on http://bordeaux.guix.gnu.org...
http://bordeaux.guix.gnu.org
  0.0% substitutes available (0 out of 1)
  unknown substitute sizes
  0.0 MiB on disk (uncompressed)
  (continuous integration information unavailable)
1
------

I expected it to return successfully because a substitute is available,
but I see that it could also make sense if "success" means that a
substitute is available from all servers.

However, the current behaviour breaks the use case of "wait until a
substitute is available and then download it", like this for example:

$ while true; do guix weather linux-libre && break; sleep 600; done; guix build 
--no-grafts linux-libre

I'm not sure if this topic was discussed previously... your thoughts?