More verbose error message

be nice to any future devs that see this error message :)
style
set app_pages order to match site

We might want to do something better in the future, like in the
metadata:

```
app_pages:
  high:
    - something_that_really_matches
  low:
    - something_that_doesnt_match_so_much
```

/cc @jrubenoff: this means that we won’t have totally granular control
of the order that knowledge base articles appear in the screendoor
dropdown.