add an error message for "unknown error"

libubus.c

@@ -36,6 +36,7 @@ const char *__ubus_strerror[__UBUS_STATUS_LAST] = {
 	[UBUS_STATUS_PERMISSION_DENIED] = "Permission denied",
 	[UBUS_STATUS_TIMEOUT] = "Request timed out",
 	[UBUS_STATUS_NOT_SUPPORTED] = "Operation not supported",
+	[UBUS_STATUS_UNKNOWN_ERROR] = "Unknown error",
 };
 
 static struct blob_buf b;

ubusmsg.h

@@ -86,6 +86,7 @@ enum ubus_msg_status {
 	UBUS_STATUS_PERMISSION_DENIED,
 	UBUS_STATUS_TIMEOUT,
 	UBUS_STATUS_NOT_SUPPORTED,
+	UBUS_STATUS_UNKNOWN_ERROR,
 	__UBUS_STATUS_LAST
 };