"not 403" and "not 404" conditions

It also looks like "~error_page" (when error_page is the name of the context) doesn't work in a "context" condition. Other "~context" specifiers work fine.

I wrote a patch to reverse the logic here. It's pretty hacky and it removes the ability to say "only show on 404 error pages", but it seems to work. Attached.

Ignore the last - here's a much nicer patch.

It lets the user pick between "An error page", "403 error pages only", "404 error pages only" or "not an error page".

One problem is that the selection is done with checkboxes, but the logic only makes sense if one and only one value is picked. The selection should really be done using a select dropdown, but I didn't know how to do that inside the Context API.