feat(authn): add templatized error page

ref CNOUS/mse#3907

internal/config/layers.go

@@ -31,6 +31,7 @@ type QueueLayerConfig struct {
 }
 
 type AuthnLayerConfig struct {
+	Debug       InterpolatedBool     `yaml:"debug"`
 	TemplateDir InterpolatedString   `yaml:"templateDir"`
 	OIDC        AuthnOIDCLayerConfig `yaml:"oidc"`
 }