Re-work event loop run() API so it can return a Result

This re-works the portable `run()` API that consumes the `EventLoop` and
runs the loop on the calling thread until the app exits.

This can be supported across _all_ platforms and compared to the
previous `run() -> !` API is now able to return a `Result` status on all
platforms except iOS and Web. Fixes: #2709

By moving away from `run() -> !` we stop calling `std::process::exit()`
internally as a means to kill the process without returning which means
it's possible to return an exit status and applications can return from
their `main()` function normally.

This also fixes Android support where an Activity runs in a thread but
we can't assume to have full ownership of the process (other services
could be running in separate threads).

Additionally all examples have generally been updated so that `main()`
returns a `Result` from `run()`

Fixes: #2709

examples/web.rs

@@ -7,7 +7,7 @@ use winit::{
     window::{Fullscreen, WindowBuilder},
 };
 
-pub fn main() {
+pub fn main() -> Result<(), impl std::error::Error> {
     let event_loop = EventLoop::new();
 
     let builder = WindowBuilder::new().with_title("A fantastic window!");
@@ -56,7 +56,7 @@ pub fn main() {
             }
             _ => (),
         }
-    });
+    })
 }
 
 #[cfg(wasm_platform)]
@@ -72,7 +72,7 @@ mod wasm {
         console_log::init_with_level(log::Level::Debug).expect("error initializing logger");
 
         #[allow(clippy::main_recursion)]
-        super::main();
+        let _ = super::main();
     }
 
     pub fn insert_canvas_and_create_log_list(window: &Window) -> web_sys::Element {