reorderbuffer: preserve errno while reporting error

Clobbering errno during cleanup after an error is an oft-repeated, easy
to make mistake.  Deal with it here as everywhere else, by saving it
aside and restoring after cleanup, before ereport'ing.

In passing, add a missing errcode declaration in another ereport() call
in the same file, which I noticed while skimming the file looking for
similar problems.

Backpatch to 9.4, where this code was introduced.

src/backend/replication/logical/reorderbuffer.c

@@ -2317,7 +2317,10 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
 	if (write(fd, rb->outbuf, ondisk->size) != ondisk->size)
 	{
+		int save_errno = errno;
+
 		CloseTransientFile(fd);
+		errno = save_errno;
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("could not write to data file for XID %u: %m",
@@ -3070,7 +3073,8 @@ ApplyLogicalMappingFile(HTAB *tuplecid_data, Oid relid, const char *fname)
 	fd = OpenTransientFile(path, O_RDONLY | PG_BINARY, 0);
 	if (fd < 0)
 		ereport(ERROR,
-				(errmsg("could not open file \"%s\": %m", path)));
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m", path)));
 
 	while (true)
 	{