pgsql: Improve new wording of libpq's connection failure messages.

Improve new wording of libpq's connection failure messages.

"connection to server so-and-so failed:" seems clearer than the
previous wording "could not connect to so-and-so:" (introduced by
52a10224e), because the latter suggests a network-level connection
failure.  We're now prefixing this string to all types of connection
failures, for instance authentication failures; so we need wording
that doesn't imply a low-level error.

Per discussion with Robert Haas.

Discussion: https://postgr.es/m/CA+TgmobssJ6rS22dspWnu-oDxXevGmhMD8VcRBjmj-b9UDqRjw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/27a48e5a16ff2227ddf44ee717d9bcd89d22a7aa

Modified Files
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src/bin/pg_dump/t/002_pg_dump.pl                   |  2 +-
.../ecpg/test/expected/connect-test5.stderr        |  4 +--
src/interfaces/ecpg/test/pg_regress_ecpg.c         |  8 +++---
src/interfaces/libpq/fe-connect.c                  | 31 +++++++++++-----------
4 files changed, 23 insertions(+), 22 deletions(-)