pgsql: Improve error messages in ltree_in and lquery_in.

Improve error messages in ltree_in and lquery_in.

Ensure that the type name is mentioned in all cases (bare "syntax error"
isn't that helpful). Avoid using the term "level", since that's not
used in the documentation. Phrase error position reports as "at
character N" not "in position N"; the latter seems ambiguous, and it's
certainly not how we say it elsewhere. For the same reason, make the
character position values be 1-based not 0-based. Provide a position
in more cases. (I continued to leave that out of messages complaining
about end-of-input, where it seemed pointless, as well as messages
complaining about overall input complexity, where fingering any one part
of the input would be arbitrary.)

Discussion: https://postgr.es/m/15582.1585529626@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e07e2a40bd0c3c02a9baf2e5ddccf665e73208fb

Modified Files
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contrib/ltree/expected/ltree.out | 13 ++-
contrib/ltree/ltree_io.c | 99 ++++++++++++----------
contrib/ltree/sql/ltree.sql | 1 +
contrib/ltree_plpython/expected/ltree_plpython.out | 2 +-
4 files changed, 63 insertions(+), 52 deletions(-)