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/macos/event_loop.rs

@@ -24,7 +24,7 @@ use raw_window_handle::{AppKitDisplayHandle, RawDisplayHandle};
 
 use super::appkit::{NSApp, NSApplicationActivationPolicy, NSEvent, NSWindow};
 use crate::{
-    error::RunLoopError,
+    error::EventLoopError,
     event::Event,
     event_loop::{ControlFlow, EventLoopClosed, EventLoopWindowTarget as RootWindowTarget},
     platform::{macos::ActivationPolicy, pump_events::PumpStatus},
@@ -148,7 +148,9 @@ impl Default for PlatformSpecificEventLoopAttributes {
 }
 
 impl<T> EventLoop<T> {
-    pub(crate) fn new(attributes: &PlatformSpecificEventLoopAttributes) -> Self {
+    pub(crate) fn new(
+        attributes: &PlatformSpecificEventLoopAttributes,
+    ) -> Result<Self, EventLoopError> {
         if !is_main_thread() {
             panic!("On macOS, `EventLoop` must be created on the main thread!");
         }
@@ -178,7 +180,7 @@ impl<T> EventLoop<T> {
 
         let panic_info: Rc<PanicInfo> = Default::default();
         setup_control_flow_observers(Rc::downgrade(&panic_info));
-        EventLoop {
+        Ok(EventLoop {
             _delegate: delegate,
             window_target: Rc::new(RootWindowTarget {
                 p: Default::default(),
@@ -186,14 +188,14 @@ impl<T> EventLoop<T> {
             }),
             panic_info,
             _callback: None,
-        }
+        })
     }
 
     pub fn window_target(&self) -> &RootWindowTarget<T> {
         &self.window_target
     }
 
-    pub fn run<F>(mut self, callback: F) -> Result<(), RunLoopError>
+    pub fn run<F>(mut self, callback: F) -> Result<(), EventLoopError>
     where
         F: FnMut(Event<T>, &RootWindowTarget<T>, &mut ControlFlow),
     {
@@ -204,12 +206,12 @@ impl<T> EventLoop<T> {
     // `pump_events` elegantly (we just ask to run the loop for a "short" amount of
     // time and so a layered implementation would end up using a lot of CPU due to
     // redundant wake ups.
-    pub fn run_ondemand<F>(&mut self, callback: F) -> Result<(), RunLoopError>
+    pub fn run_ondemand<F>(&mut self, callback: F) -> Result<(), EventLoopError>
     where
         F: FnMut(Event<T>, &RootWindowTarget<T>, &mut ControlFlow),
     {
         if AppState::is_running() {
-            return Err(RunLoopError::AlreadyRunning);
+            return Err(EventLoopError::AlreadyRunning);
         }
 
         // # Safety
@@ -287,7 +289,7 @@ impl<T> EventLoop<T> {
         if exit_code == 0 {
             Ok(())
         } else {
-            Err(RunLoopError::ExitFailure(exit_code))
+            Err(EventLoopError::ExitFailure(exit_code))
         }
     }