Allow overriding the default 500 response on uncaught errors

**Proposal:** Add a runtime hook (something like `interpretErrors`) that adds a handler that runs when an endpoint throws an error. The handler can return a `ResponseTuple` array or undefined (call next handler or send a 500 response).

```ts
interpretErrors((error, req) => {
if (req.method == 'post' && error.message == 'Unexpected end of JSON input') {
return [400, null, { json: { error: 'malformed-json' } }]
}
})
```