pgsql: pg_combinebackup: Error message improvements

pg_combinebackup: Error message improvements

Make the wordings of some file-related error messages more like those
used in other files.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c5c82123d3050c3a5eef0f51e9783f1cc5004ba0

Modified Files
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src/bin/pg_combinebackup/backup_label.c | 4 ++--
src/bin/pg_combinebackup/copy_file.c | 14 +++++++-------
src/bin/pg_combinebackup/load_manifest.c | 2 +-
src/bin/pg_combinebackup/pg_combinebackup.c | 18 +++++++++---------
src/bin/pg_combinebackup/reconstruct.c | 22 ++++++++++------------
src/bin/pg_combinebackup/write_manifest.c | 6 +++---
6 files changed, 32 insertions(+), 34 deletions(-)