Improve unit test output

The doctest output can be a little cryptic, with output related to the
doctest framework and not about the doctests themselves.  Make the
output a little clearer.

* Print entire doctest line, including code
* Print reason when fail to trigger rule

Change-Id: Iccbf1401e3a31da998fbc261bf8d20836af877d0
Reviewed-on: https://review.openstack.org/29741
Reviewed-by: Sean Dague <sean@dague.net>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins