If a device can't be opened, don't keep going but display the error

returned from open() and quit.

PR: bin/81035
Submitted by: Peter Schuller <peter.schuller at infidyne.com>
Approved by: rrs (mentor)

usr.sbin/boot0cfg/boot0cfg.c

@@ -356,6 +356,8 @@ write_mbr(const char *fname, int flags, u_int8_t *mbr, int mbr_size)
 	if (n != mbr_size)
 	   errx(1, "%s: short write", fname);
 	return;
+    } else {
+	    err(1, "write_mbr: %s", fname);
     }
 
     /*