Redirect to / if a given path doesn't exist

[sohkai]

apm-serve currently tries to fetch content provided in a package.

For example, aragon.mainnet.aragonpm.com/manifest.json fetches the deployment manifest.json, but aragon.mainnet.aragonpm.com/organization may be a common typo and should at least redirect to the index.

Given that apm-serve is only serving as a gateway to frontends deployed on aragonPM, redirecting to / (to get the index.html, if one exists) is akin to what would need to happen with SPA-based routers anyway.