pgsql: Bring pg_nextoid()'s error messages into line with message style

Bring pg_nextoid()'s error messages into line with message style guide.

Noticed while reviewing nearby code. Given all the disclaimers about
this not being meant as user-facing code, I wonder whether we should
make these non-translatable? But in any case there's little excuse
for them not to be good English.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bd5e8b627bae9e394352a388d2ad30465eafea2c

Modified Files
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src/backend/catalog/catalog.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)