check error messages in SSL tests

I noticed that a couple of test cases in the SSL tests fail to connect
not for the reason that the tests think they should.  Here is a patch to
augment the test setup so that a test for connection rejection also
checks that we get the expected error message.

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Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services