As I mentioned in my comment, you are not returning an UiApp object. See Ui Service documentation for information on how to display an UI. At a very basic level you can use the following code
function doGet(e)
{
var app = UiApp.createApplication();
var value = logActiveUserEmail();
app.add(app.createLabel('Your email address is ' + value));
// Return the UI App
return app;
}
function logActiveUserEmail()
{
var email = Session.getActiveUser().getEmail();
Logger.log(email);
return email;
}