Fix error reporting in reindexdb

When failing to reindex a table, reindexdb would generate an extra error
message related to a database failure, which is misleading.

Backpatch all the way down, as this has been introduced by 85e9a5a0.

Discussion: https://postgr.es/m/CAOBaU_Yo61RwNO3cW6WVYWwH7EYMPuexhKqufb2nFGOdunbcHw@mail.gmail.com
Author: Julien Rouhaud
Reviewed-by: Daniel Gustafsson, Álvaro Herrera, Tom Lane, Michael
Paquier
Backpatch-through: 9.4