add missing externs to repo

externs/webextension_polyfill/webnavigation/OnErrorOccurredEvent.hx

@@ -0,0 +1,18 @@
+package webextension_polyfill.webnavigation;
+
+/**
+	Fired when an error occurs and the navigation is aborted. This can happen if either a network error occurred,
+	or the user aborted the navigation.
+**/
+typedef OnErrorOccurredEvent = {
+	/**
+		Registers an event listener <em>callback</em> to an event.
+	**/
+	function addListener(callback:(details:OnErrorOccurredDetailsType) -> Void, ?filters:EventUrlFilters):Void;
+	/**
+		Deregisters an event listener <em>callback</em> from an event.
+	**/
+	function removeListener(callback:(details:OnErrorOccurredDetailsType) -> Void):Void;
+	function hasListener(callback:(details:OnErrorOccurredDetailsType) -> Void):Bool;
+	function hasListeners():Bool;
+};