pgsql: Improve error reporting in jsonpath

Improve error reporting in jsonpath

This commit contains multiple improvements to error reporting in jsonpath
including but not limited to getting rid of following things:

* definition of error messages in macros,
* errdetail() when valueable information could fit to errmsg(),
* word "singleton" which is not properly explained anywhere,
* line breaks in error messages.

Reported-by: Tom Lane
Discussion: /messages/by-id/14890.1555523005@sss.pgh.pa.us
Author: Alexander Korotkov
Reviewed-by: Tom Lane

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/29ceacc3f93720d3ebb7e7e999f8b7fe9622389c

Modified Files
--------------
src/backend/utils/adt/jsonpath_exec.c | 120 ++++++-------------
src/backend/utils/adt/jsonpath_gram.y | 6 +-
src/backend/utils/adt/jsonpath_scan.l | 23 ++--
src/test/regress/expected/jsonb_jsonpath.out | 137 ++++++++--------------
src/test/regress/expected/jsonpath.out | 61 ++++------
src/test/regress/expected/jsonpath_encoding.out | 18 +--
src/test/regress/expected/jsonpath_encoding_1.out | 12 +-
7 files changed, 130 insertions(+), 247 deletions(-)