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

src/platform_impl/macos/event_loop.rs

@@ -6,7 +6,7 @@ use std::{
     mem,
     os::raw::c_void,
     panic::{catch_unwind, resume_unwind, AssertUnwindSafe, RefUnwindSafe, UnwindSafe},
-    process, ptr,
+    ptr,
     rc::{Rc, Weak},
     sync::mpsc,
 };
@@ -192,16 +192,11 @@ impl<T> EventLoop<T> {
         &self.window_target
     }
 
-    pub fn run<F>(mut self, callback: F) -> !
+    pub fn run<F>(mut self, callback: F) -> Result<(), RunLoopError>
     where
         F: 'static + FnMut(Event<'_, T>, &RootWindowTarget<T>, &mut ControlFlow),
     {
-        let exit_code = match self.run_ondemand(callback) {
-            Err(RunLoopError::ExitFailure(code)) => code,
-            Err(_err) => 1,
-            Ok(_) => 0,
-        };
-        process::exit(exit_code);
+        self.run_ondemand(callback)
     }
 
     // NB: we don't base this on `pump_events` because for `MacOs` we can't support