Image writer: ignore partx add problems

MassStorageCloner/blkwrite.sh

@@ -57,7 +57,7 @@ if [ "x${TGT%%[0-9]*}" == "x$TGT" ]
 then
   # have a "master" block device
   echo "adding partitions for $TGT with partx"
-  partx -a "$TGT"
+  partx -a "$TGT" || true
 fi
 
 if [ x$res == x0 ]