Add interface for error handling.

src/EventErrorInterface.php

@@ -0,0 +1,31 @@
+<?php
+declare(strict_types=1);
+
+namespace Psr\Event\Dispatcher;
+
+/**
+ * An object that contains information about an error triggered by Event handling.
+ */
+interface EventErrorInterface
+{
+    /**
+     * Returns the event that triggered this error condition.
+     *
+     * @return EventInterface
+     */
+    public function getEvent() : EventInterface;
+
+    /**
+     * Returns the throwable (Exception or Error) that triggered this error condition.
+     *
+     * @return \Throwable
+     */
+    public function getThrowable() : \Throwable;
+
+    /**
+     * Returns the callable from which the exception or error was generated.
+     *
+     * @return callable
+     */
+    public function getListener() : callable;
+}