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: https://postgr.es/m/tencent_1EE1430B1E6C18A663B8990F@qq.com
Backpatch-through: 14

src/bin/pg_rewind/file_ops.c

@@ -327,7 +327,7 @@ slurpFile(const char *datadir, const char *path, size_t *filesize)
 				 fullpath);
 
 	if (fstat(fd, &statbuf) < 0)
-		pg_fatal("could not open file \"%s\" for reading: %m",
+		pg_fatal("could not stat file \"%s\" for reading: %m",
 				 fullpath);
 
 	len = statbuf.st_size;