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/exercises/sec_groups.sh b/exercises/sec_groups.sh
index a33c9c6..b73afdf 100755
--- a/exercises/sec_groups.sh
+++ b/exercises/sec_groups.sh
@@ -68,7 +68,7 @@
 
 # Delete secgroup
 nova secgroup-delete $SEC_GROUP_NAME || \
-    die "Failure deleting security group $SEC_GROUP_NAME"
+    die $LINENO "Failure deleting security group $SEC_GROUP_NAME"
 
 set +o xtrace
 echo "*********************************************************************"