pg_rewind: Improve message wording

src/bin/pg_rewind/copy_fetch.c

src/bin/pg_rewind/file_ops.c

src/bin/pg_rewind/filemap.c

src/bin/pg_rewind/libpq_fetch.c

src/bin/pg_rewind/parsexlog.c

src/bin/pg_rewind/pg_rewind.c

src/bin/pg_rewind/timeline.c