revid-cli: Report when Pi camera is not enabled

[Alan Noble]

To reproduce run revid-cli on a Pi without the camera initialized. Raspivid will produce the following error (among others):
mmal: Camera is not enabled in this build. Try running "sudo raspi-config" and ensure that "camera" has been enabled

Revid-cli does not report any errors (or warnings).

[Ella Pietraroia]

Will be addressed with issue #169