Updating a CORS header
Fixing `TurtleIO.respond()` such that it doesn't crash if middleware has terminated the response
Fixing a missing argument passed to `TurtleIO.error()` within `TurtleIO.run()`
Fixing how chained middleware errors are handled
Simplifying the execution of middleware
Changing middleware error handler such that if the `Error.message` matches an `TurtleIO.codes` key, it'll be used for the HTTP status