emit the error string if srv can't be opened.

Reference: /n/atom/patch/applied/kfssrverr
Date: Sat Feb 22 16:59:28 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/disk/kfs/main.c	Sat Feb 22 16:59:05 2014
+++ /sys/src/cmd/disk/kfs/main.c	Sat Feb 22 16:59:06 2014
@@ -376,9 +376,9 @@
 		if(fd < 0)
 			panic(s);
 	}
-	sprint(buf, "%d", sfd);
+	snprint(buf, sizeof buf, "%d", sfd);
 	if(write(fd, buf, strlen(buf)) != strlen(buf))
-		panic("srv write");
+		panic("srv write: %r");
 	return sfd;
 }