pgsql: Improve error reporting from validate_exec().

Improve error reporting from validate_exec().

validate_exec() didn't guarantee to set errno to something appropriate
after a failure, leading to callers not being able to print an on-point
message.  Improve that.

Noted by Kyotaro Horiguchi, though this isn't exactly his proposal.

Discussion: https://postgr.es/m/20220615.131403.1791191615590453058.horikyota.ntt@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/920072339f304a7da0b5de966117420c96ad78cb

Modified Files
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src/bin/pg_upgrade/exec.c | 11 ++---------
src/common/exec.c         | 16 ++++++++++++++--
2 files changed, 16 insertions(+), 11 deletions(-)