be extra explicit for people reading logs

people seem to be confused by gate_hook failed, even though 5 lines
above we say running gate_hook. Make it extra explicit what to do
next.

Change-Id: I19d77b905c61311c0c68e23ed67f137df682cf34

devstack-vm-gate-wrap.sh

@@ -307,7 +307,8 @@ GATE_RETVAL=$?
 RETVAL=$GATE_RETVAL
 
 if [ $GATE_RETVAL -ne 0 ]; then
-    echo "ERROR: gate_hook failed"
+    echo "ERROR: the main setup script run by this job failed - exit code: $GATE_RETVAL"
+    echo "    please look at the relevant log files to determine the root cause"
 fi
 
 # Run post test hook if we have one