Add facility to recognize specific error messages.

Also add the invalid user name and password message. This list should
eventually grow as more errors are recognized. It prevents a client from
having to either bubble up literally any text from RouterOS or have to
rebuild all the different error strings themselves.

error.go

@@ -12,6 +12,11 @@ var (
 	errAsyncLoopEnded = errors.New("method Async(): loop has ended - probably read error")
 )
 
+// A list of sentences we recognize from the router
+const (
+	MsgInvalidUserNameOrPassword = "invalid user name or password (6)"
+)
+
 // UnknownReplyError records the sentence whose Word is unknown.
 type UnknownReplyError struct {
 	Sentence *proto.Sentence
@@ -27,6 +32,19 @@ type DeviceError struct {
 	Sentence *proto.Sentence
 }
 
+// Well-known errors. We could just dynamically create these out of the various
+// strings we get back from RouterOS, but by listing them here we capture knowledge
+// and experience recognizing the different messages. This makes it easier for clients
+// to write good error-handling code.
+var (
+	ErrInvalidAuthentication =  errors.New(MsgInvalidUserNameOrPassword)
+)
+
+// A mapping between the recognized sentences and error types
+var ErrorsByMessage = map[string]error{
+	MsgInvalidUserNameOrPassword: ErrInvalidAuthentication,
+}
+
 func (err *DeviceError) fetchMessage() string {
 	if m := err.Sentence.Map["message"]; m != "" {
 		return m
@@ -38,3 +56,15 @@ func (err *DeviceError) fetchMessage() string {
 func (err *DeviceError) Error() string {
 	return fmt.Sprintf("from RouterOS device: %s", err.fetchMessage())
 }
+
+// Given a sentence see if it's a well-known error. If it is, return that error.
+// Otherwise produce a new DeviceError with the details of the sentence.
+// Users are encouraged to send pull-requests with new error types to make error
+// handling by type easier.
+func DeviceErrorFromSentence(sen* proto.Sentence) error {
+	m := sen.Map["message"]
+	if err, ok := ErrorsByMessage[m]; ok {
+		return err
+	}
+	return &DeviceError{sen}
+}

reply.go

@@ -26,6 +26,7 @@ func (r *Reply) String() string {
 	return sb.String()
 }
 
+// Return whether or not we are done processing, and any error detected in the sentence
 func (r *Reply) processSentence(sen *proto.Sentence) (bool, error) {
 	switch sen.Word {
 	case reSentence:
@@ -34,7 +35,7 @@ func (r *Reply) processSentence(sen *proto.Sentence) (bool, error) {
 		r.Done = sen
 		return true, nil
 	case trapSentence, fatalSentence:
-		return sen.Word == fatalSentence, &DeviceError{sen}
+		return sen.Word == fatalSentence, DeviceErrorFromSentence(sen)
 	case "":
 		// API docs say that empty sentences should be ignored
 	default: