Alamofire error is always nil, while request fails - swift

Question

In case of 400(bad request) or 403(unauthorized) I can grab info about fail only from response, while error,passed in param is always nil - do it needs any extra setup?
Alamofire.request(Router.SignIn(emailField.text, passwordField.text)).response { (request, response, data, error) in
println(error)
println(NSString(data: data, encoding: NSUTF8StringEncoding))
}
UPD: as mattt advised added valide like this:
Alamofire.request(UdacityRouter.SignIn(emailField.text, passwordField.text)).validate().response
in
println(error)
println(NSString(data: data, encoding: NSUTF8StringEncoding))
}
As result I received "Optional("The operation couldn’t be completed. (com.alamofire.error error -1.)")" What is far away from meaningfull explanation of failure.. I wonder why I can't simply get error with a simple failure explanation as AFNetworking does..

Answer 1

Need to add .validate() because of by default, Alamofire treats any completed request to be successful, regardless of the content of the response.
From Alamofire documentation:
Validation
By default, Alamofire treats any completed request to be successful,
regardless of the content of the response. Calling validate before a
response handler causes an error to be generated if the response had
an unacceptable status code or MIME type.
Manual Validation
Alamofire.request(.GET, "http://httpbin.org/get", parameters: ["foo": "bar"])
.validate(statusCode: 200..<300)
.validate(contentType: ["application/json"])
.response { _, _, _, error in
println(error)
}
Automatic Validation
Automatically validates status code within 200...299 range, and that
the Content-Type header of the response matches the Accept header of
the request, if one is provided.
Alamofire.request(.GET, "http://httpbin.org/get", parameters: ["foo": "bar"])
.validate()
.response { _, _, _, error in
println(error)
}