bug#31974: If a phase returns #f, a warning is issued, but the build continues

[Mark H Weaver]

I just noticed that I made a mistake in commit
d8a3b1b9e847d4a44d2695f95af77170d4d2788f, which changed 'gnu-build' in
(guix build gnu-build-system) to issue a warning if a phase returns a
value other than #t.

The result is that if a phase returns a value other than #t, a warning
is issued, but the build nonetheless continues to the next phase, and
the build could ultimately "succeed" even some phases returned #f.

In other words, the return values of phases are effectively ignored,
except for the warning, so the only way to cause a build with phases to
fail is to raise an exception.

We can't fix this problem without a full rebuild.  Sorry about that :-(

The only good news here is that I've already fixed all of the core
packages to return #t from all phases and to report errors using
exceptions.  Ditto for all of the packages that I use on my GNOME
desktop.  The remaining packages that might return #f from a phase could
be fixed in 'master'.

      Mark