trivial: platform: add string for NM_PLATFORM_ERROR_NOT_SLAVE

src/platform/nm-platform.c

@@ -175,6 +175,8 @@ nm_platform_get_error_msg (void)
 		return "object not found";
 	case NM_PLATFORM_ERROR_EXISTS:
 		return "object already exists";
+	case NM_PLATFORM_ERROR_NOT_SLAVE:
+		return "link not a slave";
 	default:
 		return "invalid error number";
 	}