Propagate error from EventLoop creation

Inner panics could make it hard to trouble shoot the issues and for some
users ints not desirable.

The inner panics were left only when they are used to `assert!` during
development.

examples/multithreaded.rs

@@ -17,7 +17,7 @@ fn main() -> Result<(), impl std::error::Error> {
     const WINDOW_SIZE: PhysicalSize<u32> = PhysicalSize::new(600, 400);
 
     SimpleLogger::new().init().unwrap();
-    let event_loop = EventLoop::new();
+    let event_loop = EventLoop::new().unwrap();
     let mut window_senders = HashMap::with_capacity(WINDOW_COUNT);
     for _ in 0..WINDOW_COUNT {
         let window = WindowBuilder::new()