build-chroot: provide more debugging info

When the build chroot fails to install, print out some of the
debootstrap output, so that more information is available when the
build fails only in CI.

build-chroot/create.sh

@@ -30,7 +30,12 @@ then
 fi
 
 mkdir -m 'u=rwx,go=rx' "${HOMEWORLD_CHROOT}"
-sudo debootstrap --include="$(grep -vE '^#' packages.list | tr '[:space:]' '\n' | sed '/^$/d' | tr '\n' ,)" stretch "${HOMEWORLD_CHROOT}" http://debian.csail.mit.edu/debian/
+if ! sudo debootstrap --include="$(grep -vE '^#' packages.list | tr '[:space:]' '\n' | sed '/^$/d' | tr '\n' ,)" stretch "${HOMEWORLD_CHROOT}" http://debian.csail.mit.edu/debian/
+then
+    echo "debootstrap failed; printing log" 1>&2
+    cat "${HOMEWORLD_CHROOT}/debootstrap/debootstrap.log"
+    exit 1
+fi
 # TODO: build our own Bazel, rather than grabbing it from Google's repo
 ./install-bazel.sh --root "${HOMEWORLD_CHROOT}"
 sudo chroot "${HOMEWORLD_CHROOT}" groupadd "$(id -gn)" -g "$(id -g)"