Refactor error logging

It is hard to grep errors in current log. so in this patch,
I'm updating die function which also writes log for
screen_log_dir/error.log.
In future, we may categolize negative fault by using
this error.log.

Change-Id: I70a8cfe67ed408284f5c88c762c6bb8acb8ecdb2
diff --git a/lib/quantum_plugins/openvswitch b/lib/quantum_plugins/openvswitch
index 726c6c3..288fa69 100644
--- a/lib/quantum_plugins/openvswitch
+++ b/lib/quantum_plugins/openvswitch
@@ -49,9 +49,7 @@
         # REVISIT - also check kernel module support for GRE and patch ports
         OVS_VERSION=`ovs-vsctl --version | head -n 1 | awk '{print $4;}'`
         if [ $OVS_VERSION \< "1.4" ] && ! is_service_enabled q-svc ; then
-            echo "You are running OVS version $OVS_VERSION."
-            echo "OVS 1.4+ is required for tunneling between multiple hosts."
-            exit 1
+            die $LINENO "You are running OVS version $OVS_VERSION. OVS 1.4+ is required for tunneling between multiple hosts."
         fi
         iniset /$Q_PLUGIN_CONF_FILE OVS enable_tunneling True
         iniset /$Q_PLUGIN_CONF_FILE OVS local_ip $HOST_IP