OO.ui.alert() and OO.ui.confirm() should "queue" the dialogs, rather than failing if a dialog is already open (they will return a rejected promise and do nothing).

OO.ui.alert( 'Foo.' );
OO.ui.confirm( 'Foo?' );

OO.ui.confirm( 'Foo?' );
OO.ui.alert( 'Foo.' );

Current behavior: the first dialog opens, the second dialog never opens.

Expected behavior: the first dialog opens, the second dialog opens after the user closes the first dialog.