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
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/fee476da952a1f02f7ccf6e233fb4824c2bf6af4

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