config: add new error type

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

config/config.go

@@ -23,6 +23,8 @@ var (
 	ErrInvalidStruct = errors.New("invalid struct specified")
 	// ErrWatcherStopped is returned when source watcher has been stopped
 	ErrWatcherStopped = errors.New("watcher stopped")
+	// ErrWatcherNotImplemented returned when config does not implement watch
+	ErrWatcherNotImplemented = errors.New("watcher not implemented")
 )
 
 // Config is an interface abstraction for dynamic configuration

config/default.go

@@ -2,7 +2,6 @@ package config
 
 import (
 	"context"
-	"fmt"
 	"reflect"
 	"strconv"
 	"strings"
@@ -271,7 +270,7 @@ func (c *defaultConfig) Name() string {
 }
 
 func (c *defaultConfig) Watch(ctx context.Context, opts ...WatchOption) (Watcher, error) {
-	return nil, fmt.Errorf("not implemented")
+	return nil, ErrWatcherNotImplemented
 }
 
 // NewConfig returns new default config source