[Android] Add logging to base_test_results.

BUG=615157

Review-Url: https://codereview.chromium.org/2179493002
Cr-Commit-Position: refs/heads/master@{#407169}

build/android/pylib/base/base_test_result.py

@@ -180,7 +180,8 @@ class TestRunResults(object):
     Args:
       results: An instance of TestRunResults.
     """
-    assert isinstance(results, TestRunResults)
+    assert isinstance(results, TestRunResults), (
+           'Expected TestRunResult object: %s' % type(results))
     with self._results_lock:
       # pylint: disable=W0212
       self._results.update(results._results)