fix bad usage of 'su'.  instead use 'set -e'.

I had had tried to use -e argument to su, thinking that it would
go through to sh, but it does not.  instead, just use 'set -e'
diff --git a/stack.sh b/stack.sh
index a4cbc2e..2d76875 100755
--- a/stack.sh
+++ b/stack.sh
@@ -69,9 +69,9 @@
     cp -r -f "$PWD" "$STACK_DIR"
     chown -R stack "$STACK_DIR"
     if [[ "$SHELL_AFTER_RUN" != "no" ]]; then
-        exec su -ec "cd $STACK_DIR; bash stack.sh; bash" stack
+        exec su -c "set -e; cd $STACK_DIR; bash stack.sh; bash" stack
     else
-        exec su -ec "cd $STACK_DIR; bash stack.sh" stack
+        exec su -c "set -e; cd $STACK_DIR; bash stack.sh" stack
     fi
     exit 1
 fi