rebaseline_server unittests: output tkdiff lines when actuals don't match expectations

gm/rebaseline_server/base_unittest.py

Index: gm/rebaseline_server/base_unittest.py
diff --git a/gm/rebaseline_server/base_unittest.py b/gm/rebaseline_server/base_unittest.py
index 172a8c0ed913853f63157f6bf59ae77e8008e61f..eec0c8da0a4e836a97392482e2054baf02ad59c1 100755
--- a/gm/rebaseline_server/base_unittest.py
+++ b/gm/rebaseline_server/base_unittest.py
@@ -49,8 +49,11 @@ class TestCase(unittest.TestCase):
       # Ravi notes: if somebody later comes along and adds cleanup code below
       # this assert, then if tests fail, the artifacts will not be cleaned up.
       assert (not different_files), \
-        ('found differing files between actual dir %s and expected dir %s: %s' %
-         (self._output_dir_actual, self._output_dir_expected, different_files))
+        ('found differing files:\n' +
+         '\n'.join(['tkdiff %s %s &' % (
+             os.path.join(self._output_dir_actual, basename),
+             os.path.join(self._output_dir_expected, basename))
+                    for basename in different_files]))
 
   def shortDescription(self):
     """Tell unittest framework to not print docstrings for test cases."""