Give less scary error messages when a containerised command fails

This affects errors encountered at build time and at system-integration time.

New errors look like this:

    ERROR: Command failed: baserock/system-integration/02-install-gerrit-gerrit-installation-binaries-misc-0000:
    Containerisation settings: {'mounts': (('dev/shm', 'tmpfs', 'none'), ('tmp', 'tmpfs', 'none')), 'mount_proc': True, 'root': '/var/tmp/staging/tmp1YQ2yN/minimal-system-x86_64-generic.inst'}
    Error output:
    + install -D /usr/share/gerrit/gerrit-2.9.war /home/gerrit2/gerrit/gerrit-2.9.war -o gerrit2 -g gerrit2 -m 644
    install: can't change ownership of /home/gerrit2/gerrit/gerrit-2.9.war: Operation not permitted

Previously the error message would have been this:

    Command failed: unshare --mount -- sh -ec.
    mount --make-rprivate /
    root="$1"
    shift
    while true; do
        case "$1" in
        --)
            shift
            break
            ;;
        *)
            mount_point="$1"
            mount_type="$2"
            mount_source="$3"
            shift 3
            path="$root/$mount_point"
            mount -t "$mount_type" "$mount_source" "$path"
            ;;
        esac
    done

    exec "$@"
     - /var/tmp/staging/tmppeA1Iw/gerrit-x86_64.inst/ dev/shm tmpfs none tmp tmpfs none -- linux-user-chroot --chdir . --mount-proc proc
    /var/tmp/staging/tmppeA1Iw/gerrit-x86_64.inst/ baserock/system-integration/02-install-gerrit-gerrit-installation-binaries-misc-0000
    + install -D /usr/share/gerrit/gerrit-2.9.war /home/gerrit2/gerrit/gerrit-2.9.war -o gerrit2 -g gerrit2 -m 644
    install: can't change ownership of /home/gerrit2/gerrit/gerrit-2.9.war: Operation not permitted

0 files changed, 0 insertions, 0 deletions