pgsql: Make multixact error message more explicit

Make multixact error message more explicit

There are recent reports involving a very old error message that we have
no history of hitting -- perhaps a recently introduced bug.  Improve the
error message in an attempt to improve our chances of investigating the
bug.

Per reports from Dimos Stamatakis and Bob Krier.

Backpatch to 11.

Discussion: https://postgr.es/m/CO2PR0801MB2310579F65529380A4E5EDC0E20A9@CO2PR0801MB2310.namprd08.prod.outlook.com
Discussion: https://postgr.es/m/17518-04e368df5ad7f2ee@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2cf41cd30997f6a79fb962007a3cd11edcf9d013

Modified Files
--------------
src/backend/access/transam/multixact.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)