pgsql: Minor robustness improvements for isolationtester.

Minor robustness improvements for isolationtester.

Notice and complain about PQcancel() failures. Also, don't dump core if
an error PGresult doesn't contain severity and message subfields, as it
might not if it was generated by libpq itself. (We have a longstanding
TODO item to improve that, but in the meantime isolationtester had better
cope.)

I tripped across the latter item while investigating a trouble report on
buildfarm member spoonbill. As for the former, there's no evidence that
PQcancel failure is actually involved in spoonbill's problem, but it still
seems like a bad idea to ignore an error return code.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e084b1446ffa9ec09c3774221011ed686a8b1e7d

Modified Files
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src/test/isolation/isolationtester.c | 31 ++++++++++++++++++++++---------
1 files changed, 22 insertions(+), 9 deletions(-)