How to make a step down DC-DC converter fail safe?
The simple fact is that a failure to short on the power switching element (the usual failure mode of semiconductors) or a failure of the control circuitry can effectively put full input voltage on the load. These are single-point failures and will probably result in destruction of the load. 

You could use an electronic overcurrent/overvoltage controller such as the LTC4361: 



This chip isn't particularly cheap (about $3.50), but it will work with either a P-channel or N-channel MOSFET (internal boost for the N-channel, obviously). It can provide overvoltage protection up to 85V and overcurrent protection limited by the MOSFET. The sense resistor drops only 50mV so it's pretty low-loss. 

This kind of circuit works by breaking the connection between power supply and expensive load. 

The other (perhaps more) popular approach is to use a fuse (or polyfuse) and a crowbar circuit to short the output so the load (LEDs in your case) are not damaged while the fuse has enough I^2T to do its thing and open up the circuit. An SCR is often used as the crowbar power switching element.