Make Application and Sandbox return a Result

src/sandbox.rs

@@ -1,5 +1,6 @@
+use crate::executor;
 use crate::{
-    executor, Application, Color, Command, Element, Settings, Subscription,
+    Application, Color, Command, Element, Error, Settings, Subscription,
 };
 
 /// A sandboxed [`Application`].
@@ -64,7 +65,7 @@ use crate::{
 /// ```no_run
 /// use iced::{Element, Sandbox, Settings, Text};
 ///
-/// pub fn main() {
+/// pub fn main() -> iced::Result {
 ///     Hello::run(Settings::default())
 /// }
 ///
@@ -159,7 +160,7 @@ pub trait Sandbox {
     /// It should probably be that last thing you call in your `main` function.
     ///
     /// [`Sandbox`]: trait.Sandbox.html
-    fn run(settings: Settings<()>)
+    fn run(settings: Settings<()>) -> Result<(), Error>
     where
         Self: 'static + Sized,
     {