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
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b20e381422089f66d2cfa7a6286c8053006d3afb

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