Install an error template for the dashboard.

This isn't all that informative of an error message, but it does let you
know that (A) your action was acknowledged, and (B) it resulted in an
error.  Previously, (A) wasn't even clear.
Dashboard: GET /history will always give you an answer.

If no records exist for the given URL, we now return [] with status code
200.  This is probably a better interpretation of what we had before:
we're saying "I acknowledge the existence of your resource (a URL) and I
have zero history records that relate to it".