When a framework is configured, but not on disk, the BT continue with theme errors

[mauritslamers]

Whenever a framework is missing on disk, but is configured, the BT will give a warning, but continue. It will then generate a series of warnings that the configured theme cannot be found, and then start the server.

Whenever a framework is missing, the app builder should simply abort building the app, and if no apps remain, stop the server.