Rewrite error handling as middleware

[mwpastore]

Sinja is jumping through too many hoops to play nicely with Sinatra's built-in error-handling in conjunction with sideloading and UDFs. Let's:

• Make halt raise errors (as it does currently)
• Enable :raise_errors
• Move error normalization and serialization logic to middleware
  • Does an internal call pass through middleware?
• Don't raise from middleware
• Allow disabling middleware for testing

Other notes:

• Make exception config options register error handlers