Add facility to recognize specific error messages.

The initial list is quite short, incorporating just known errors of
invalid username/passwords, incorrect login, and 'entry already exists'.
This list should eventually grow as more errors are recognized.

By adding in these types we prevent a client from having to either bubble
up literally any text from RouterOS or have to rebuild all the different
error strings themselves.

client_test.go

@@ -183,9 +183,7 @@ func TestInvalidLogin(t *testing.T) {
 
 	require.Error(t, err, "dial succeeded; want error")
 
-	var devErr *DeviceError
-	require.Truef(t, errors.As(err, &devErr), "wait for device error: %v", err)
-	require.Contains(t, []string{"cannot log in", "invalid user name or password (6)"}, devErr.fetchMessage())
+	require.Equal(t, ErrInvalidUserNameOrPassword, err)
 }
 
 func TestTrapHandling(tt *testing.T) {

error.go

@@ -27,6 +27,19 @@ type DeviceError struct {
 	Sentence *proto.Sentence
 }
 
+type decodedDeviceError string
+
+// 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 (
+	ErrInvalidUserNameOrPassword = decodedDeviceError("invalid user name or password (6)")
+	ErrIncorrectLogin            = decodedDeviceError("incorrect login")
+	ErrAlreadyExists             = decodedDeviceError("failure:entry already exists")
+)
+
+func (e decodedDeviceError) Error() string { return string(e) }
 func (err *DeviceError) fetchMessage() string {
 	if m := err.Sentence.Map["message"]; m != "" {
 		return m

proto_test.go

@@ -77,9 +77,8 @@ func TestLoginIncorrectPre643(t *testing.T) {
 	err := c.Login("userTest", "passTest")
 	require.Error(t, err, "Login succeeded; want error")
 
-	var top *DeviceError
-	require.Truef(t, errors.As(err, &top), "want=DeviceError, have=%#v", err)
-	require.Contains(t, []string{"incorrect login"}, top.fetchMessage())
+	require.Truef(t, errors.Is(err, ErrIncorrectLogin),
+		"want=ErrIncorrectLogin, have=%#v", err)
 }
 
 func TestLoginIncorrectPost643(t *testing.T) {
@@ -96,9 +95,8 @@ func TestLoginIncorrectPost643(t *testing.T) {
 	err := c.Login("userTest", "passTest")
 	require.Error(t, err, "Login succeeded; want error")
 
-	var top *DeviceError
-	require.Truef(t, errors.As(err, &top), "want=DeviceError, have=%#v", err)
-	require.Contains(t, []string{"invalid user name or password (6)"}, top.fetchMessage())
+	require.Truef(t, errors.Is(err, ErrInvalidUserNameOrPassword),
+		"want=ErrInvalidUserNameOrPassword, have=%#v", err)
 }
 
 func TestLoginNoChallenge(t *testing.T) {

reply.go

@@ -1,6 +1,7 @@
 package routeros
 
 import (
+	"errors"
 	"strings"
 
 	"github.com/go-routeros/routeros/v3/proto"
@@ -26,6 +27,7 @@ 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 +36,11 @@ func (r *Reply) processSentence(sen *proto.Sentence) (bool, error) {
 		r.Done = sen
 		return true, nil
 	case trapSentence, fatalSentence:
-		return sen.Word == fatalSentence, &DeviceError{sen}
+		if msg, ok := sen.Map["message"]; ok {
+			// custom error found
+			return sen.Word == fatalSentence, errors.Join(&DeviceError{Sentence: sen}, decodedDeviceError(msg))
+		}
+		return sen.Word == fatalSentence, &DeviceError{Sentence: sen}
 	case "":
 		// API docs say that empty sentences should be ignored
 	default: