Issue/Knowing when we've requested a record that doesn't exist

AKA: Correct generation of 404s

The problem: Users will use invalid IDs in requests because of errors, and because records disappear. With our current template driven system we will always return an about record, and always redirect to a specific encoding/version, whether or not it exists.

Possible solutions:

• Coordinate with providers to keep up to date lists of identifiers and build checks of validity into our system
• Record, as metadata, information about how a client can tell that the record doesn't exist. For encodings, we could, for instance, record a pattern that matches the providers "not found" page.