Static error pages have incorrect base tag

[dnsl48]

Somewhat relevant: silverstripe/silverstripe-errorpage#20

However, we may need to remove the static error pages altogether from snapshots so that they may be regenerated later on on demos.

[chillu]

The effect is that any "page not found" or "server error" page looks unstyled, and throws security issues. If we fix the server errors, that page won't appear, but we should definitely fix it for the "page not found" case anyway