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