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/web.rs

@@ -8,7 +8,7 @@ use winit::{
 };
 
 pub fn main() -> Result<(), impl std::error::Error> {
-    let event_loop = EventLoop::new();
+    let event_loop = EventLoop::new().unwrap();
 
     let builder = WindowBuilder::new().with_title("A fantastic window!");
     #[cfg(wasm_platform)]