Error out when the provided configuration file doesn't exist

[vivien]

Currently qt-kiosk-browser silently falls back to the built-in URL value if it fails to load the configuration file passed as argument like this:

qt-kiosk-browser sdflkjsdflkjsdflkj

This can lead to unwanted behavior like this, thus qt-kiosk-browser must error out with an explicit message if it fails to load the user provided configuration file.

[otavio]

Can you prepare a PR for this?