Include simple, well-formatted default error pages

[amcclain]

We should ship our base image with one or two simple, modern looking error pages for nginx to serve in the case of a 404 or 500 (or whatever other nginx-level error codes we think likely).

This would be a nice bit of polish over the stock nginx error pages, which IMO always make it look like you don't know what you're doing if you let them through.

This looks like the kind of approach I am envisioning (note did not test or vet this blog, just a top search hit but it's basically what I want):

https://blog.adriaan.io/one-nginx-error-page-to-rule-them-all.html