Show error messages when Firebase operations fail

Consolidating https://github.com/popcodeorg/popcode/issues/1005 https://github.com/popcodeorg/popcode/issues/1004 and a related case of a project that never saved.

Firebase generally tries to be pretty resilient to bad network connectivity, but this seems to have the effect of obscuring situations in which it’s entirely unable to connect to the database. There is existing error handling code for most/all Firebase operations but the library itself doesn’t seem to trigger errors as one might expect.

So, we should figure out how to get an up-or-down on Firebase’s ability to successfully complete an operation, and show an error message in these situations (possibly incomplete list):

• Loading a snapshot fails because of a database connection error (this definitely shouldn’t just load a blank project!!)
• Loading existing projects fails because of a database connection error
• Auto-saving to the user’s account fails because of a database connection error
• Creating a snapshot fails because of a database connection error

Commencez par suivre la gestion des erreurs Firebase existante pour les quatre opérations indiquées : charger un snapshot, charger les projets existants, effectuer une sauvegarde automatique et créer un snapshot. Déterminez comment détecter de manière fiable les opérations de base de données ayant échoué et afficher une erreur au lieu de charger silencieusement un projet vide ; le travail est considéré comme terminé lorsque chaque échec indiqué est signalé à l’utilisateur.