pgsql: Improve libpq's error reporting for SSL failures.

Improve libpq's error reporting for SSL failures.

In many cases, pqsecure_read/pqsecure_write set up useful error messages,
which were then overwritten with useless ones by their callers. Fix this
by defining the responsibility to set an error message to be entirely that
of the lower-level function when using SSL.

Back-patch to 8.3; the code is too different in 8.2 to be worth the
trouble.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/276164125c801117a1237043b1212878e1a9bac2

Modified Files
--------------
src/interfaces/libpq/fe-misc.c | 24 ++++++++++----
src/interfaces/libpq/fe-secure.c | 61 +++++++++++++++++++++++++++++++-------
2 files changed, 67 insertions(+), 18 deletions(-)