security_NetworkListeners: better error message and comments

Improve error message on test failure, 'Baseline mismatch' is not very
helpful.

Add comment to explain 'baseline' and 'baseline.arc' files after review
feedback

BUG=chromium:620781
TEST=ran test on cyan-cheets laptop

Change-Id: Ide1e7b7a41d6a0b12892709658074a3f3c287007
Reviewed-on: https://chromium-review.googlesource.com/356420
Commit-Ready: Nicolas Norvez <norvez@chromium.org>
Tested-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
diff --git a/client/site_tests/security_NetworkListeners/security_NetworkListeners.py b/client/site_tests/security_NetworkListeners/security_NetworkListeners.py
index e8235ed..05f5d53 100644
--- a/client/site_tests/security_NetworkListeners/security_NetworkListeners.py
+++ b/client/site_tests/security_NetworkListeners/security_NetworkListeners.py
@@ -26,7 +26,9 @@
 _LSOF_NODE = -3
 _LSOF_NAME = -2
 
-
+# Open ports on ARC-enabled test firmwares are different from the non-ARC case
+# These files provide a whitelist of services expected to listen in each case
+# (ARC and non-ARC)
 _BASELINE_DEFAULT_NAME = 'baseline'
 _BASELINE_ARC_NAME = 'baseline.arc'
 
@@ -134,4 +136,4 @@
 
             # Only fail if there's unexpected listeners.
             if new_listeners:
-                raise error.TestFail('Baseline mismatch')
+                raise error.TestFail('Found unexpected network listeners')