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

Comienza rastreando el manejo de errores existente de Firebase para las cuatro operaciones indicadas: cargar un snapshot, cargar proyectos existentes, guardar automáticamente y crear un snapshot. Determina cómo detectar de forma fiable las operaciones de base de datos fallidas y mostrar un error en lugar de cargar silenciosamente un proyecto vacío; se considera terminado cuando cada fallo indicado se muestra al usuario.