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/nova b/lib/nova
index 849ec57..3749790 100644
--- a/lib/nova
+++ b/lib/nova
@@ -542,8 +542,7 @@
     screen_it n-api "cd $NOVA_DIR && $NOVA_BIN_DIR/nova-api"
     echo "Waiting for nova-api to start..."
     if ! wait_for_service $SERVICE_TIMEOUT http://$SERVICE_HOST:$service_port; then
-      echo "nova-api did not start"
-      exit 1
+      die $LINENO "nova-api did not start"
     fi
 
     # Start proxies if enabled