pgsql: Report libpq errors correctly if session setup or teardown steps

Report libpq errors correctly if session setup or teardown steps fail in
isolation regression tests.

Alvaro committed these fixes to master branch on Tue Jul 29th, as part of
Noah Misch's patch. The rest of that patch is not needed on 9.1, but this
part should be backpatched.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b936d9264c08418909e3ec5d6107bd104d62fdc2

Modified Files
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src/test/isolation/isolationtester.c |    4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)