Print a warning instead of a trace if the build bulb is not plugged in

Pretty simple... here's some example output.

```
Finished in 0.54901 seconds
5 examples, 3 failures

Failed examples:

rspec ./spec/models/device.rb:54 # Device should remove staff from the access list
rspec ./spec/models/device.rb:28 # Device should not add a staff member from another customer
rspec ./spec/models/device.rb:38 # Device should block access to staff that are not on the access list

WARNING: Couldn't find build bulb

Randomized with seed 35939
```