pgsql: pg_upgrade: issue helpful error message for use on standbys

pg_upgrade:  issue helpful error message for use on standbys

Commit 777e6ddf1723306bd2bf8fe6f804863f459b0323 checked for a shut down
message from a standby and allowed it to continue.  This patch reports a
helpful error message in these cases, suggesting to use rsync as
documented.

Diagnosed-by: Martín Marqués

Discussion: https://postgr.es/m/CAPdiE1xYCow-reLjrhJ9DqrMu-ppNq0ChUUEvVdxhdjGRD5_eA@mail.gmail.com

Backpatch-through: 9.3

Branch
------
REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bd30f51c0ec97207ac9baf68d03fd4e851f9777f

Modified Files
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src/bin/pg_upgrade/controldata.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)