Something like:

class Rack::BadRequest < StandardError
  def initialize(message, status = 400)
    super(message)
    @status = status
  end
  attr :status
end

This class would be used to indicate failures within Rack, and servers should be able to catch it and correctly map it to a failed response.

There are places in the multipart parsing code and Rack::Request where various conditions can be the result of a bad client HTTP request. It feels like it would be useful to have an exception that indicates that. The current situation right now, is we may raise an IOError or some other exception which becomes a 500 Internal Server Error, which while correct in a way (the server basically blew up), doesn't really reflect the nature of the "request being invalid" - e.g. a bad request, preconditions failed, etc.

See #1994 (comment) for an example use case.