Print the stream name in the error message if dup2(2) fails.

src/vdfp_pipe.c

@@ -145,7 +145,8 @@ mk_dup(int oldfd, int newfd)
 {
 	errno = 0;
 	if (dup2(oldfd, newfd) == -1) {
-		fprintf(stderr, "dup2(2) failed: %s", vstrerror(errno));
+		fprintf(stderr, "dup2(2) failed for %s: %s", stream_name[newfd],
+			vstrerror(errno));
 		return (-1);
 	}
 	return (0);