bug#24140: Substitute hash mismatches not properly reported

[ludo@gnu.org (Ludovic Courtès)]

Hello,

When using ‘guix package’, upon a substitute hash mismatch, all we see
is something like this:

--8<---------------cut here---------------start------------->8---
Found valid signature for /gnu/store/cpw9yca1mcqhqfq450dr3rz2jzr95l69-glib-2.48.0
From https://mirror.hydra.gnu.org/nar/cpw9yca1mcqhqfq450dr3rz2jzr95l69-glib-2.48.0
Downloading cpw9yc...-glib-2.48.0 (13.5MiB installed)...
 glib-2.48.0                                                                                    744KiB/s 00:04 | 2.9MiB transferred
killing process 11821
guix system: error: build failed: some substitutes for the outputs of derivation `/gnu/store/lkvlm17z8qm1v6r4n5ja5vcmsp7d860i-graphviz-2.38.0.drv' failed (usually happens due to networking issues); try `--fallback' to build derivation from source 
--8<---------------cut here---------------end--------------->8---

The error message from build.cc:

--8<---------------cut here---------------start------------->8---
      if (expectedHash != actualHash)
          throw SubstError(format("hash mismatch in downloaded path `%1%': expected %2%, got %3%")
              % storePath % printHash(expectedHash) % printHash(actualHash));
--8<---------------cut here---------------end--------------->8---

… is only visible when we set-build-options #:print-build-trace? #t
(like ‘guix build’ does, but ‘guix package’ and others do not.)

The message should always be displayed, regardless of
#:print-build-trace?.

Ludo’.