Improve HTTP responses when the server is panicking

Currently, there are cases where AdGuard Home's internals encounter a runtime panic, which results in the frontend receiving no responses. AGH doesn't crash itself, since the default HTTP server has its own panic handling, but that's still rather poor UX.

What we need is a simple HTTP middleware that:

1. Recovers the panic.

2. Writes a `500` response with an appropriate content type (typically, a JSON object) that contains the panic's message.

3. Writes a backtrace, preferably on the `error` level as opposed to the current `debug`.