Make sure unittest fails if doctest has failures

[karenc]

Even if README examples failed, unittest was passing:

**********************************************************************
1 items had failures:
  29 of  62 in README.rst
***Test Failed*** 29 failures.
ok

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Ran 1 test in 6.217s

OK

This PR changes this to:

**********************************************************************
1 items had failures:
  29 of  62 in README.rst
***Test Failed*** 29 failures.
FAIL

======================================================================
FAIL: test_readme (cnxpublishing.tests.test_doctests.DocTestTestCase)
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Traceback (most recent call last):
  File "/home/karen/cnx-publishing/cnxpublishing/tests/test_doctests.py", line 59, in test_readme
    self.fail('DocTest failed: {}'.format(results))
AssertionError: DocTest failed: TestResults(failed=29, attempted=62)

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Ran 1 test in 6.221s

FAILED (failures=1)