[Telemetry] Make measurement smoke test error more useful

BUG=
NOTRY=True

Review URL: https://codereview.chromium.org/191973003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255849 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/measurements/measurement_unittest.py

@@ -56,4 +56,5 @@ class MeasurementUnitTest(unittest.TestCase):
       logging.info('running: %s', benchmark)
       options = options_for_unittests.GetCopy()
       options.output_format = 'none'
-      self.assertEqual(0, SinglePageBenchmark().Run(options))
+      self.assertEqual(0, SinglePageBenchmark().Run(options),
+                       msg='Failed: %s' % benchmark)