Camera isn't available - Trusted Face ( Android Lollipop - Nexus 5)
I believe that this is an issue with the Smart Lock app itself. When I enabled the face unlock I started getting this error. The camera ceases to work after some time, when I notice that the smart lock icon just blinks on the lock screen (which shows a failed attempt to access camera). Then neither the camera app works nor can I access the camera via Smart Lock settings. I started having this issue whenever I enabled face recognition and I stopped having this issue altogether when I disabled the said feature.

It's a shame that despite being an extremely efficient and fast feature, this causes such a problem that it needs to be disabled.