pgsql: Standardize error reports in unimplemented I/O functions.

Standardize error reports in unimplemented I/O functions.

We chose a specific wording of the not-implemented errors for
pseudotype I/O functions and other cases where there's little
value in implementing input and/or output.  gtsvectorin never
got that memo though, nor did most of contrib.  Make these all
fall in line, mostly because I'm a neatnik but also to remove
unnecessary translatable strings.

gbtreekey_in needs a bit of extra love since it supports
multiple SQL types.  Sadly, gbtreekey_out doesn't have the
ability to do that, but I think it's unreachable anyway.

Noted while surveying datatype input functions to see what we
have left to fix.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d02ef65bce6575e10ac0b4e7b8552ff67687c944

Modified Files
--------------
contrib/btree_gist/btree_gist.c   | 18 +++++++++++-------
contrib/hstore/hstore_gist.c      | 14 ++++++++++----
contrib/intarray/_intbig_gist.c   | 10 ++++++----
contrib/ltree/ltree_gist.c        | 10 ++++++----
contrib/pg_trgm/trgm_gist.c       | 14 ++++++++++----
src/backend/utils/adt/tsgistidx.c |  6 ++++--
6 files changed, 47 insertions(+), 25 deletions(-)