pgsql: Fix some error message inconsistencies

Fix some error message inconsistencies

These errors are very unlikely going to show up, but in the event that
they happen, some incorrect information would have been provided:
- In pg_rewind, a stat() failure was reported as an open() failure.
- In pg_combinebackup, a check for the new directory of a tablespace
mapping was referred as the old directory.
- In pg_combinebackup, a failure in reading a source file when copying
blocks referred to the destination file.

The changes for pg_combinebackup affect v17 and newer versions. For
pg_rewind, all the stable branches are affected.

Author: Man Zeng <zengman@halodbtech.com>
Discussion: /messages/by-id/tencent_1EE1430B1E6C18A663B8990F@qq.com
Backpatch-through: 14

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/74a116a79b47631e163c9814f39f5d218834e94c

Modified Files
--------------
src/bin/pg_combinebackup/copy_file.c | 2 +-
src/bin/pg_combinebackup/pg_combinebackup.c | 2 +-
src/bin/pg_rewind/file_ops.c | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)