Intentionally handling a not found error in pass through request

[nlively]

I've set up a proxy to redirect a wildcard for my blog. Would like to have some intentional handling around a 404 error if the path I'm passing through doesn't have a destination and gets a 404.

What's the best way to go about this?

config.middleware.insert(0, Rack::ReverseProxy) do
  reverse_proxy /^\/blog(\/?.*)$/, 'http://blog.mysite.com$1'
end

In this example, let's say the following are valid:

• /blog (goes to http://blog.mysite.com)
• /blog/my-article (goes to http://blog.mysite.com/my-article)

But the following is NOT valid

• /blog/does-not-exist (gets a 404 from http://blog.mysite.com/does-not-exist)

I'd like to catch that error and redirect to a page in my Rails app (not something behind the reverse proxy). Is this possible? and how?