Translate error type to NSError.
Interface type is not supported yet.
Change-Id: I54abba2360cff41ef8ca08063b0120e7edd65a47
Reviewed-on: https://go-review.googlesource.com/10793
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>