Propagate error from EventLoop creation

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

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

This reverts commit 9f91bc413fe20618bd7090829832bb074aab15c3 which
reverted the original patch which was merged without a proper review.

Fixes: #500.

src/platform_impl/ios/event_loop.rs

@@ -19,11 +19,9 @@ use objc2::rc::Id;
 use objc2::ClassType;
 use raw_window_handle::{RawDisplayHandle, UiKitDisplayHandle};
 
-use super::uikit::{UIApplication, UIApplicationMain, UIDevice, UIScreen};
-use super::view::WinitUIWindow;
-use super::{app_state, monitor, view, MonitorHandle};
 use crate::{
     dpi::LogicalSize,
+    error::EventLoopError,
     event::Event,
     event_loop::{
         ControlFlow, EventLoopClosed, EventLoopWindowTarget as RootEventLoopWindowTarget,
@@ -31,6 +29,10 @@ use crate::{
     platform::ios::Idiom,
 };
 
+use super::uikit::{UIApplication, UIApplicationMain, UIDevice, UIScreen};
+use super::view::WinitUIWindow;
+use super::{app_state, monitor, view, MonitorHandle};
+
 #[derive(Debug)]
 pub(crate) enum EventWrapper {
     StaticEvent(Event<Never>),
@@ -75,7 +77,9 @@ pub struct EventLoop<T: 'static> {
 pub(crate) struct PlatformSpecificEventLoopAttributes {}
 
 impl<T: 'static> EventLoop<T> {
-    pub(crate) fn new(_: &PlatformSpecificEventLoopAttributes) -> EventLoop<T> {
+    pub(crate) fn new(
+        _: &PlatformSpecificEventLoopAttributes,
+    ) -> Result<EventLoop<T>, EventLoopError> {
         assert_main_thread!("`EventLoop` can only be created on the main thread on iOS");
 
         static mut SINGLETON_INIT: bool = false;
@@ -93,7 +97,7 @@ impl<T: 'static> EventLoop<T> {
         // this line sets up the main run loop before `UIApplicationMain`
         setup_control_flow_observers();
 
-        EventLoop {
+        Ok(EventLoop {
             window_target: RootEventLoopWindowTarget {
                 p: EventLoopWindowTarget {
                     receiver,
@@ -101,7 +105,7 @@ impl<T: 'static> EventLoop<T> {
                 },
                 _marker: PhantomData,
             },
-        }
+        })
     }
 
     pub fn run<F>(self, event_handler: F) -> !