pgsql: Rework some error strings for REINDEX CONCURRENTLY with system c

Rework some error strings for REINDEX CONCURRENTLY with system catalogs

This makes the whole user experience more consistent when bumping into
failures, and more in line with the rewording done via 508300e.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera
Discussion: /messages/by-id/20190514153252.GA22168@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/20e1cc898da6f82b5680973c107e8a96d46caf01

Modified Files
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src/backend/commands/indexcmds.c | 10 ++++------
src/test/regress/expected/create_index.out | 12 ++++++------
2 files changed, 10 insertions(+), 12 deletions(-)