Issue Description:

A new URL error was introduced, NSURLErrorAppTransportSecurityRequiresSecureConnection, but was never added to the Swift Foundation overlay. As a result, if you have an error with this code, casting to URLError and accessing the code property crashes. Given that we'll probably see other new error codes in the future, we probably shouldn't crash in this case, because even if we always update the Foundation overlay, that doesn't help apps compiled against an older SDK.

Sample program:

import Foundation

let nserr = NSError(domain: NSURLErrorDomain, code: NSURLErrorAppTransportSecurityRequiresSecureConnection, userInfo: nil)
let urlerr = nserr as! URLError
print("code:", urlerr.code)

This crashes on the access for urlerr.code.

One possible solution would be to introduce a new case other(rawValue: Int) case to handle any unknown codes. We'd also need to implement == on URLError.Code at this point as we'd lose the default impl. An alternative solution is just to default to .unknown for unknown codes, but that means throwing away the actual code which isn't great.