Make 404 pages package-specific

[nex3]

The default 404 page is used for all 404 pages, including those for packages whose documentation failed to build. Since the server knows the package and version that the user is requesting, it would be nice if it would look up the corresponding build log and include a link (or better, render it inline) along with instructions about how to fix the issue.

[adam-singer]

@nex3 will look into this. All content is served up from Cloud Storage. I think we could make the 404.html page more lively by running some client side script code to look at the query url and fetch the logs.

[nex3]

Another possibility would be to upload a custom error page for every build that fails.

[adam-singer]

@nex3 I do like that idea. This would also be related to the following issue #54

Downside is it would require a full build of failed packages to be done. Could script around this by doing a custom datastore query and adding those packages to a queued build. Doing it all in one shot.