Make PostgresNode.pm's poll_query_until() more chatty about failures.

Reporting only the stderr is unhelpful when the problem is that the
server output we're getting doesn't match what was expected.  So we
should report the query output too; and just for good measure, let's
print the query we used and the output we expected.

Back-patch to 9.5 where poll_query_until was introduced.

Discussion: https://postgr.es/m/17913.1539634756@sss.pgh.pa.us