Provide CustomNSError default implementation

Pull default CustomNSError implementation from https://github.com/apple/swift/blob/master/stdlib/public/SDK/Foundation/NSError.swift

Author: zayass

Foundation/NSError.swift

@@ -274,6 +274,38 @@ public protocol CustomNSError : Error {
     var errorUserInfo: [String : Any] { get }
 }
 
+public extension CustomNSError {
+    /// Default domain of the error.
+    static var errorDomain: String {
+        return String(reflecting: self)
+    }
+
+    /// The error code within the given domain.
+    var errorCode: Int {
+        return _swift_getDefaultErrorCode(self)
+    }
+
+    /// The default user-info dictionary.
+    var errorUserInfo: [String : Any] {
+        return [:]
+    }
+}
+
+extension 
+where Self: RawRepresentable, Self.RawValue: SignedInteger {
+    // The error code of Error with integral raw values is the raw value.
+    public var errorCode: Int {
+        return numericCast(self.rawValue)
+    }
+}
+
+extension CustomNSError where Self: RawRepresentable, Self.RawValue: UnsignedInteger {
+    // The error code of Error with integral raw values is the raw value.
+    public var errorCode: Int {
+        return numericCast(self.rawValue)
+    }
+}
+
 public extension Error where Self : CustomNSError {
     /// Default implementation for customized NSErrors.
     var _domain: String { return Self.errorDomain }