No clear error when GPG signature errors

[jelly]

Rebuilding ant shows some GPG errors and systemd-nspawn exit's. No clear error is however shown when repro exits.

error: 'cache/java-environment-common-3-1-any.pkg.tar.xz': invalid or corrupted package (PGP signature)
error: 'cache/java-runtime-common-3-1-any.pkg.tar.xz': invalid or corrupted package (PGP signature)
warning: downgrading package json-c (0.14-2 => 0.13.1-2)

[Foxboron]

There is no exit code from pacman, so we either need to parse the output or do something else.