Show the error dialog when requests to PUT power levels fail

This is why I dislike Q.

src/components/views/rooms/MemberInfo.js

@@ -376,6 +376,7 @@ module.exports = WithMatrixClient(React.createClass({
                 // get out of sync if we force setState here!
                 console.log("Power change success");
             }, function(err) {
+                var ErrorDialog = sdk.getComponent("dialogs.ErrorDialog");
                 Modal.createDialog(ErrorDialog, {
                     title: "Failure to change power level",
                     description: err.message