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/web/event_loop/mod.rs

@@ -1,16 +1,18 @@
+use std::marker::PhantomData;
+
+use crate::error::EventLoopError;
+use crate::event::Event;
+use crate::event_loop::{ControlFlow, EventLoopWindowTarget as RootEventLoopWindowTarget};
+
+use super::{backend, device, window};
+
 mod proxy;
 pub(crate) mod runner;
 mod state;
 mod window_target;
 
-pub use self::proxy::EventLoopProxy;
-pub use self::window_target::EventLoopWindowTarget;
-
-use super::{backend, device, window};
-use crate::event::Event;
-use crate::event_loop::{ControlFlow, EventLoopWindowTarget as RootEventLoopWindowTarget};
-
-use std::marker::PhantomData;
+pub use proxy::EventLoopProxy;
+pub use window_target::EventLoopWindowTarget;
 
 pub struct EventLoop<T: 'static> {
     elw: RootEventLoopWindowTarget<T>,
@@ -20,13 +22,13 @@ pub struct EventLoop<T: 'static> {
 pub(crate) struct PlatformSpecificEventLoopAttributes {}
 
 impl<T> EventLoop<T> {
-    pub(crate) fn new(_: &PlatformSpecificEventLoopAttributes) -> Self {
-        EventLoop {
+    pub(crate) fn new(_: &PlatformSpecificEventLoopAttributes) -> Result<Self, EventLoopError> {
+        Ok(EventLoop {
             elw: RootEventLoopWindowTarget {
                 p: EventLoopWindowTarget::new(),
                 _marker: PhantomData,
             },
-        }
+        })
     }
 
     pub fn run<F>(self, mut event_handler: F) -> !