SR-2683 URLSessionDelegate didCompleteWithError callback never triggered

[seabaylea]

SR-2638 reports that the following URLSessionDelegate callback is never triggered:

func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?)

This is actually caused by a function signature issue - I'm not sure why the compiler isn't picking it up.

On Linux, the function signature for the callback is currently using the old style:

internal func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: NSError?)

ie. its specified as an NSError instead of Error.

This PR switches over the API, and coerces the NSError to an Error before calling the callback.

[parkera]

@swift-ci please test and merge