feat(authn): add templatized error page

ref CNOUS/mse#3907

internal/proxy/director/layer/authn/layer_options.go

@@ -17,6 +17,7 @@ type LayerOptions struct {
 
 type TemplatesOptions struct {
 	Forbidden TemplateOptions `mapstructure:"forbidden"`
+	Error     TemplateOptions `mapstructure:"error"`
 }
 
 type TemplateOptions struct {
@@ -43,6 +44,9 @@ func DefaultLayerOptions() LayerOptions {
 			Forbidden: TemplateOptions{
 				Block: "default",
 			},
+			Error: TemplateOptions{
+				Block: "default",
+			},
 		},
 	}