Show error messages when Firebase operations fail

[outoftime]

Consolidating #1005 #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