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.