[SE-0112] Make NSURLError embed its NSError.

This ensures that we don't lose information about the NSError when it
is bridged to NSError. Use this to expose information for common keys
in the NSURLError domain (NSURLErrorFailingURLErrorKey,
NSURLErrorFailingURLStringErrorKey,
NSURLErrorFailingURLPeerTrustErrorKey).

This is effectively part of bullet (4) in the proposed solution of
SE-0112, applied to NSURLError. There are a handful of other domains
that need this treatment as well.

Author: DougGregor