Add an indicator that shows up in dev mode if an exception is thrown

[bitspittle]

Right now, we hope that users will have their console tab open when something goes wrong, but if they don't check it, it can just look like their page doesn't load or stops responding.

This indicator should only show up in dev mode and there should be a way to configure it to turn it off, since it will have to intercept and rethrow any exception it gets, and it might put up UI that interferes with the user's own UI.

Otherwise, this should be implemented similar to the "Building..." spinner. It should probably be code written straight into the index.html script directly in raw JS.

There's room to discuss design, but I'm imagining a little red icon button that temporarily appears in the bottom right corner with a number on it (representing exception count since last cleared). When clicked, it would open up a larger window with a history of exceptions on it.