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

まず、リストされている4つの操作（スナップショットの読み込み、既存プロジェクトの読み込み、自動保存、スナップショットの作成）について、既存の Firebase のエラーハンドリングを追跡します。データベース操作の失敗を確実に検出し、空のプロジェクトを何も表示せずに読み込むのではなくエラーを表示する方法を確認します。リストされている各失敗がユーザーに通知されれば完了です。