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/event_loop.rs

@@ -18,7 +18,7 @@ use std::time::{Duration, Instant};
 #[cfg(wasm_platform)]
 use web_time::{Duration, Instant};
 
-use crate::error::RunLoopError;
+use crate::error::EventLoopError;
 use crate::{event::Event, monitor::MonitorHandle, platform_impl};
 
 /// Provides a way to retrieve events from the system and from the windows that were registered to
@@ -88,23 +88,23 @@ impl<T> EventLoopBuilder<T> {
     /// ***For cross-platform compatibility, the [`EventLoop`] must be created on the main thread,
     /// and only once per application.***
     ///
-    /// Attempting to create the event loop on a different thread, or multiple event loops in
-    /// the same application, will panic. This restriction isn't
-    /// strictly necessary on all platforms, but is imposed to eliminate any nasty surprises when
-    /// porting to platforms that require it. `EventLoopBuilderExt::any_thread` functions are exposed
-    /// in the relevant [`platform`] module if the target platform supports creating an event loop on
-    /// any thread.
-    ///
     /// Calling this function will result in display backend initialisation.
     ///
+    /// ## Panics
+    ///
+    /// Attempting to create the event loop off the main thread will panic. This
+    /// restriction isn't strictly necessary on all platforms, but is imposed to
+    /// eliminate any nasty surprises when porting to platforms that require it.
+    /// `EventLoopBuilderExt::any_thread` functions are exposed in the relevant
+    /// [`platform`] module if the target platform supports creating an event
+    /// loop on any thread.
+    ///
     /// ## Platform-specific
     ///
-    /// - **Linux:** Backend type can be controlled using an environment variable
-    ///   `WINIT_UNIX_BACKEND`. Legal values are `x11` and `wayland`.
-    ///   If it is not set, winit will try to connect to a Wayland connection, and if that fails,
-    ///   will fall back on X11. If this variable is set with any other value, winit will panic.
-    /// - **Android:** Must be configured with an `AndroidApp` from `android_main()` by calling
-    ///     [`.with_android_app(app)`] before calling `.build()`.
+    /// - **Wayland/X11:** to prevent running under `Wayland` or `X11` unset `WAYLAND_DISPLAY`
+    ///                    or `DISPLAY` respectively when building the event loop.
+    /// - **Android:** must be configured with an `AndroidApp` from `android_main()` by calling
+    ///     [`.with_android_app(app)`] before calling `.build()`, otherwise it'll panic.
     ///
     /// [`platform`]: crate::platform
     #[cfg_attr(
@@ -116,17 +116,17 @@ impl<T> EventLoopBuilder<T> {
         doc = "[`.with_android_app(app)`]: #only-available-on-android"
     )]
     #[inline]
-    pub fn build(&mut self) -> EventLoop<T> {
+    pub fn build(&mut self) -> Result<EventLoop<T>, EventLoopError> {
         if EVENT_LOOP_CREATED.swap(true, Ordering::Relaxed) {
-            panic!("Creating EventLoop multiple times is not supported.");
+            return Err(EventLoopError::RecreationAttempt);
         }
 
         // Certain platforms accept a mutable reference in their API.
         #[allow(clippy::unnecessary_mut_passed)]
-        EventLoop {
-            event_loop: platform_impl::EventLoop::new(&mut self.platform_specific),
+        Ok(EventLoop {
+            event_loop: platform_impl::EventLoop::new(&mut self.platform_specific)?,
             _marker: PhantomData,
-        }
+        })
     }
 
     #[cfg(wasm_platform)]
@@ -269,20 +269,14 @@ impl EventLoop<()> {
     ///
     /// [`EventLoopBuilder::new().build()`]: EventLoopBuilder::build
     #[inline]
-    pub fn new() -> EventLoop<()> {
+    pub fn new() -> Result<EventLoop<()>, EventLoopError> {
         EventLoopBuilder::new().build()
     }
 }
 
-impl Default for EventLoop<()> {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
 impl<T> EventLoop<T> {
     #[deprecated = "Use `EventLoopBuilder::<T>::with_user_event().build()` instead."]
-    pub fn with_user_event() -> EventLoop<T> {
+    pub fn with_user_event() -> Result<EventLoop<T>, EventLoopError> {
         EventLoopBuilder::<T>::with_user_event().build()
     }
 
@@ -315,7 +309,7 @@ impl<T> EventLoop<T> {
     /// [`ControlFlow`]: crate::event_loop::ControlFlow
     #[inline]
     #[cfg(not(all(wasm_platform, target_feature = "exception-handling")))]
-    pub fn run<F>(self, event_handler: F) -> Result<(), RunLoopError>
+    pub fn run<F>(self, event_handler: F) -> Result<(), EventLoopError>
     where
         F: FnMut(Event<T>, &EventLoopWindowTarget<T>, &mut ControlFlow),
     {