pgsql: Improve jsonb cast error message

Improve jsonb cast error message

Initial variant of error message didn't follow style of another casting error
messages and wasn't informative. Per gripe from Robert Haas.

Reviewer: Tom Lane
Discussion:
https://www.postgresql.org/message-id/flat/CA%2BTgmob08StTV9yu04D0idRFNMh%2BUoyKax5Otvrix7rEZC8rMw%40mail.gmail.com#CA+Tgmob08StTV9yu04D0idRFNMh+UoyKax5Otvrix7rEZC8rMw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cb5d94295986298af666534970f9bb3715574646

Modified Files
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src/backend/utils/adt/jsonb.c       | 71 ++++++++++++++++++++++++-------------
src/test/regress/expected/jsonb.out |  8 ++---
2 files changed, 51 insertions(+), 28 deletions(-)