Make grab_md5 more robust.

If $DI_CODENAME is ever set to a bad value, one would end up trying to
use a non-existing debian-installer, resulting in non-bootable images
way later. To detect that much earlier, make sure all locations
(computed from $CODENAME and $DI_CODENAME) are directories.

tools/grab_md5

@@ -8,6 +8,8 @@
 # Parse Packages and Sources files out of a mirror and pre-process
 # them into a single list ready for mkisofs to check later
 
+set -e
+
 MIRROR=$1
 ARCHES="$2"
 CODENAME=$3
@@ -19,6 +21,13 @@ do
     LOCATIONS="$MIRROR/dists/$CODENAME/ $MIRROR/dists/$DI_CODENAME/"
     echo "Looking in $LOCATIONS"
 
+    for LOCATION in $LOCATIONS; do
+        if [ ! -d $LOCATION ]; then
+            echo "Error: $LOCATION is not a directory"
+            exit 1
+        fi
+    done
+
     case $ARCH in
         source)
             FILES=`find $LOCATIONS -name Sources.gz`