* IBM-Swift/Kitura#1042 added getNSError(Error) method

since there is bridging from Error on NSError on Mac and there is no
bridging on Linux

* IBM-Swift/Kitura#1042 make getNSError receive Error?

* IBM-Swift/Kitura#1042 fixed FoundationAdapterNonLinux.getNSError

* IBM-Swift/Kitura#1042 added API comments to the public functions