Backport error-checking on determining snap bases

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/1828500

Signed-off-by: Iain Lane <iain.lane@canonical.com>

debian/changelog

@@ -1,3 +1,11 @@
+livecd-rootfs (2.525.28) bionic; urgency=medium
+
+  [ Tobias Koch ]
+  * Do proper error checking when calling snap-tool info to determine
+    the base of a snap. (LP: #1828500)
+
+ -- Tobias Koch <tobias.koch@canonical.com>  Tue, 16 Jul 2019 11:27:10 +0200
+
 livecd-rootfs (2.525.27) bionic; urgency=medium
 
   * Backport improvements to snap seeding from trunk.  LP: #1831675.

live-build/functions

@@ -445,12 +445,20 @@ _snap_preseed() {
             ;;
         *)
             # Determine if and what core snap is needed
-            local core_snap=$(/usr/share/livecd-rootfs/snap-tool info \
+            local snap_info
+
+            snap_info=$(/usr/share/livecd-rootfs/snap-tool info \
                 --cohort-key="${COHORT_KEY:-}" \
-                --channel="$CHANNEL" "$SNAP_NAME" | \
-                grep '^base:' | awk '{print $2}'
+                --channel="$CHANNEL" "${SNAP_NAME}" \
             )
 
+            if [ $? -ne 0 ]; then
+                echo "Failed to retrieve base of $SNAP_NAME!"
+                exit 1
+            fi
+
+            local core_snap=$(echo "$snap_info" | grep '^base:' | awk '{print $2}')
+
             # If $core_snap is not the empty string then SNAP itself is not a core
             # snap and we must additionally seed the core snap.
             if [ -n "$core_snap" ]; then