network_3GIdentifiers: Improve logging.

When a property fails to match, log the label on which match failed along with
the different values obtained.

BUG=none
TEST=Run network_3GIdentifiers.
Change-Id: If43e81036c170c34bb0a32be6c8c4bc9c2e544c5
Reviewed-on: https://chromium-review.googlesource.com/178707
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/client/site_tests/network_3GIdentifiers/network_3GIdentifiers.py b/client/site_tests/network_3GIdentifiers/network_3GIdentifiers.py
index eae082e..7c60d48 100644
--- a/client/site_tests/network_3GIdentifiers/network_3GIdentifiers.py
+++ b/client/site_tests/network_3GIdentifiers/network_3GIdentifiers.py
@@ -31,8 +31,8 @@
         """Validates a specific identifier by matching the values reported by
            Shill and ModemManager as well as verifying its length."""
         if device_value != modem_value:
-            message = 'Shill value "%s" does not match MM value "%s"' % \
-                      (device_value, modem_value)
+            message = ('Shill value "%s" for "%s" does not match MM value "%s"'
+                       % (device_value, label, modem_value))
             logging.error(message)
             raise error.TestFail(message)
         if (len(device_value) < min_length or len(device_value) > max_length):