pgsql: pg_upgrade: prevent check on live cluster from generating error

pg_upgrade: prevent check on live cluster from generating error

Previously an inaccurate but harmless error was generated when running
--check on a live server before reporting the servers as compatible.
The fix is to split error reporting and exit control in the exec_prog()
API.

Reported-by: Daniel Westermann

Backpatch-through: 10

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1776c817c7ef452fb47d915d1b550cd73a318944

Modified Files
--------------
src/bin/pg_upgrade/dump.c | 2 +-
src/bin/pg_upgrade/exec.c | 14 ++++++--------
src/bin/pg_upgrade/parallel.c | 9 ++++-----
src/bin/pg_upgrade/pg_upgrade.c | 24 ++++++++++++------------
src/bin/pg_upgrade/pg_upgrade.h | 6 +++---
src/bin/pg_upgrade/server.c | 18 +++++++++---------
6 files changed, 35 insertions(+), 38 deletions(-)