Better error handling

Currently we play it safe: if there is an error while rendering the page in Otter, it just outputs a blank index.html and lets the client give it a shot. An unhelpful traceback is barfed out in the console.

We can do better than this. We should have some tools for printing tracebacks in the browser when in development, perhaps other debugging tools (remote web inspector?). At the very least we should make a note that there was an error in `window.otter` so the client knows that it needs to recover.

The issue names no files, tests, or entry points. Start by locating the rendering error path and how the generated index.html and window.otter state reach the client; define the development traceback behavior and client recovery marker before implementation.

Written by the indexing model from the issue text.