Manage failed book initialisation for Standalone

When initialisation of a BakerBook fails (e.g. because of a JSON parsing error) the book will be skipped and an error will be returned, instead of having the whole application crash later down the line.
Manage failed book initialisation for Newsstand

When initialisation of a BakerBook fails (e.g. because of a JSON parsing error) the reading operation will be aborted and an error will be displayed, instead of having the whole application crash.