Encapsulate picamera exceptions into recorder_picamera.PicameraRecorder

[iingram]

Assuming we merge #101, it might make sense to move the dependency on picamera.exc out of dencam.py into recorder_picamera.PicameraRecorder and create mechanics that still share the exception in a way that dencam.py can still see and respond to but that is intended to be agnostic to camera type (in case we ultimately make dencam.py able to handle all hardware configuration and obviate the need for different top-level application modules for different hardware) and also just adhere to the principle of having dependencies encapsulated down away from the high-level code as far as reasonable/possible.