Improve error messages of input functions for pg_dependencies and pg_ndistinct

Improve error messages of input functions for pg_dependencies and pg_ndistinct

The error details updated in this commit can be reached in the
regression tests.  They did not follow the project style, and they
should be written them as full sentences.

Some of the errors are switched to use an elog(), for cases that involve
paths that cannot be reached based on the previous state of the parser
processing the input data (array start, object end, etc.).  The error
messages for these cases use now a more consistent style across the
board, with the state of the parser reported for debugging.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Author: Michael Paquier <michael@paquier.xyz>
Co-authored-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://postgr.es/m/1353179.1764901790@sss.pgh.pa.us