pgsql: Improve some JSON error messages.

Improve some JSON error messages.

These messages are new in 9.4, which hasn't been released yet, so
back-patch to REL9_4_STABLE.

Daniele Varrazzo

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/930e8ad738f5c78abd667626ca86f2553f36356f

Modified Files
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src/backend/utils/adt/json.c       |   19 ++++++++++++-------
src/test/regress/expected/json.out |    3 ++-
2 files changed, 14 insertions(+), 8 deletions(-)