Test Harness failure message for mismatched widget count not very helpful

[dobey]

This is the current failure message when testing the Preview and the widget count does not match the expected set of widgets:

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FAIL: tests.test_apps.AppsTest.test_removable_app_preview
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Traceback (most recent call last):
  File "/tmp/buildd/unity-scope-click-0.1.1+15.04.20150326bzr329pkg0vivid34/tests/scope-harness/tests/test_apps.py", line 306, in test_removable_app_preview
    self.assertMatchResult(match)
  File "/usr/lib/python3/dist-packages/scope_harness/testing.py", line 34, in assertMatchResult
    self.assertTrue(match_result.success, msg=match_result.concat_failures)
  File "/usr/lib/python3.4/unittest/case.py", line 654, in assertTrue
    raise self.failureException(msg)
AssertionError: False is not true : Failed expectations:
Incorrect number of preview widgets 8, expected 9

This is not very helpful for determining what exactly failed in the test. It would be much more helpful if a list of the actual missing widgets was output, similar to what happens when those widgets have different types than are expected. Not knowing which widgets exactly were missing from the result set make it very difficult to debug such test failures.