Modernize error message for malformed B-Tree tuple posting

Hi!

Today, while routinely fixing corruptions reported by our monitoring,
I observed a message:

2026-02-15 01:38:33.060
MSK,"<cut>","<cut>",1725745,"localhost:51330",6990f425.1a5531,1,"UPDATE",2026-02-15
01:16:05 MSK,59/1760774,183431134,ERROR,XX000,"posting list tuple with
3 items cannot be split at offset 20",,,,,,"UPDATE .... <there was
origin query,cut>,,,"","client backend",,3082007398573165030

So, two things are bothering me here. First, this is XX000 and not
XX002 which I cannot find a good reason to. Second, Index name is not
present in the error message.

Another about-corruption error messages seems to be different, for example:

ERROR,XX002,"table tid from new index tuple (58084,119) overlaps with
invalid duplicate tuple at offset 62 of block 181 in index ""<cut
name>""",,,,,,"UPDATE <cut query>

So, v1 changes the errcode for ERR_INDEX_CORRUPTION. For including
index name, we need
to pass relation to _bt_swap_posting function, which I did not do in
v1, because I'm not sure changing is worth it.

WDYT?

--
Best regards,
Kirill Reshke