Put back default strings on dialogs

But make them work by calling _t in render rather than
getDefaultProps().

Also sort out some 'Warning!' strings

src/ContentMessages.js

@@ -356,7 +356,6 @@ class ContentMessages {
                 Modal.createDialog(ErrorDialog, {
                     title: _t('Upload Failed'),
                     description: desc,
-                    button: _t("OK"),
                 });
             }
         }).finally(() => {