Replace warning by error when camera calibration file is not present

[jim-bcom]

In #5, a missing calibration file only output a warning in the logcat.
The application was running, but no AR was happening.
Maybe we should:

• treat this as an error instead (since this is critical for the application to work correctly)
• handle the error to surface it to the user in the GUI (at least a hint he should check the logcat for an error message)