improve comment

stack.sh

@@ -173,7 +173,10 @@ SCHEDULER=${SCHEDULER:-nova.scheduler.simple.SimpleScheduler}
 if [ ! -n "$HOST_IP" ]; then
     HOST_IP=`LC_ALL=C /sbin/ifconfig eth0 | grep -m 1 'inet addr:'| cut -d: -f2 | awk '{print $1}'`
     if [ "$HOST_IP" = "" ]; then
-        echo "Could not determine host ip address.  Please specify HOST_IP in your localrc."
+        echo "Could not determine host ip address."
+        echo "If this is not your first run of stack.sh, it is "
+        echo "possible that nova moved your eth0 ip address to the FLAT_NETWORK_BRIDGE."
+        echo "Please specify your HOST_IP in your localrc."
         exit 1
     fi
 fi