Added error reporting for failed subdevice initialization.

The error was encountered on Win32 when *only* the Camera subdevice
had a proper driver installed. This caused a silent failure.

Signed-off-by: JD Cole <jd@jdc.me>
Merge pull request #270 from jdcole/partial_driver_install_error

Added error reporting for failed subdevice initialization (@jdcole)

Reviewed-by: Benn Snyder <benn.snyder@gmail.com>