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/multithreaded.rs

@@ -1,7 +1,7 @@
 #![allow(clippy::single_match)]
 
 #[cfg(not(wasm_platform))]
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     use std::{collections::HashMap, sync::mpsc, thread, time::Duration};
 
     use simple_logger::SimpleLogger;