pgsql: Improve error reporting for jsonpath .double() method

Improve error reporting for jsonpath .double() method

When jsonpath .double() method detects that numeric or string can't be
converted to double precision, it throws an error.  This commit makes these
errors explicitly express the reason of failure.

Discussion: https://postgr.es/m/CAPpHfdtqJtiSXkP7tOXez18NxhLUH_-75bL8%3DOce4Ki%2Bbv7V6Q%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Tom Lane
Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3ec5f6b53dfb6acfe66a952edef1b6887da91777

Modified Files
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src/backend/utils/adt/jsonpath_exec.c        | 4 ++--
src/test/regress/expected/jsonb_jsonpath.out | 8 +++++---
src/test/regress/sql/jsonb_jsonpath.sql      | 1 +
3 files changed, 8 insertions(+), 5 deletions(-)