Show better message when there is no index.html

[aomarks]

We currently just show a text "Playground not file" response when there is no index.html file. Maybe we could display an overlay error in the same style as the one pictured here: #224 that tells the user they need to create an index.html

[justinfagnani]

I think this would be a little more clear with an address bar as in #278

Then a 404 would make sense for any HTML file not found. We could add a better message based on the files in the project, like "create an index.html file" or "go to foo.html".