Don't fail to start application if LED file doesn't exist

[fhunleth]

Right now, if you make a typo with the LED name (or any other error), the :delux app won't start and that means that any app that depends on it won't start. This has the side effect of making some Nerves applications not boot or not complete enough of the boot process to be minimally usable. There are probably arguments here about the structure of those applications that cause this issue, but it seems better to handle configuration errors in a different way.

I think the best way now is to not set up the LED and log the error. Then whenever the application tries to set the LED, Delux should log an error again (this might be done already - I didn't check). That will make it so noisy that it will be hard to miss even if you're not looking at LEDs.