pgsql: Be more robust when strerror() doesn't give a useful result.

Be more robust when strerror() doesn't give a useful result.

Back-patch commits 8e68816cc2567642c6fcca4eaac66c25e0ae5ced and
8dace66e0735ca39b779922d02c24ea2686e6521 into the stable branches.
Buildfarm testing revealed no great portability surprises, and it
seems useful to have this robustness improvement in all branches.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3eb777671b942001e3f8c740a607348f62df038d

Modified Files
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src/backend/utils/error/elog.c | 187 +++++++++++++++++++++++++++++++++++++++-
1 file changed, 183 insertions(+), 4 deletions(-)