pgsql: Convert hstore_in to report errors softly.

Convert hstore_in to report errors softly.

The error reporting here was not only old and crufty, but untested.
I took the opportunity to bring the messages into some sort of
compliance with our message style guidelines.

Discussion: https://postgr.es/m/6B6A5C77-60AD-4A71-9F3A-B2C026A281A6@dunslane.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/197f98a848233f2d8d8e7f7fd7dde3e49adaaaa3

Modified Files
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contrib/hstore/expected/hstore.out |  34 +++++++++++
contrib/hstore/hstore_io.c         | 116 ++++++++++++++++++++++++++++---------
contrib/hstore/sql/hstore.sql      |  11 ++++
3 files changed, 133 insertions(+), 28 deletions(-)