Add a test/example of how to explore the exception tree

tests/integration/issue-1376.phpt

@@ -0,0 +1,90 @@
+--CREDITS--
+Alexandre Gomes Gaigalas <alganet@gmail.com>
+--FILE--
+<?php
+
+declare(strict_types=1);
+
+require 'vendor/autoload.php';
+
+use Respect\Validation\Exceptions\NestedValidationException;
+use Respect\Validation\Exceptions\RecursiveExceptionIterator;
+use Respect\Validation\Validator as v;
+
+$input = (object) [
+    'author' => 'foo',
+];
+
+$postValidator = v::attribute('title', v::length(2, 3)->stringType())
+            ->attribute('description', v::stringType())
+            ->attribute('author', v::intType()->length(1, 2))
+            ->attribute('user', v::intVal()->length(1, 2));
+try {
+    $postValidator->assert($input);
+} catch (NestedValidationException $e) {
+    echo $e->getFullMessage();
+    echo PHP_EOL;
+    $explorer = new RecursiveIteratorIterator(
+        new RecursiveExceptionIterator($e),
+        RecursiveIteratorIterator::SELF_FIRST
+    );
+    echo PHP_EOL;
+    foreach ($explorer as $innerException) {
+        var_dump([
+            'level' => $explorer->getDepth(),
+            'message' => $innerException->getMessage(),
+        ]);
+    }
+}
+?>
+--EXPECT--
+- All of the required rules must pass for `[object] (stdClass: { "author": "foo" })`
+  - Attribute title must be present
+  - Attribute description must be present
+    - All of the required rules must pass for author
+      - author must be of type integer
+      - author must have a length between 1 and 2
+  - Attribute user must be present
+
+array(2) {
+  ["level"]=>
+  int(0)
+  ["message"]=>
+  string(31) "Attribute title must be present"
+}
+array(2) {
+  ["level"]=>
+  int(0)
+  ["message"]=>
+  string(37) "Attribute description must be present"
+}
+array(2) {
+  ["level"]=>
+  int(0)
+  ["message"]=>
+  string(30) "Attribute author must be valid"
+}
+array(2) {
+  ["level"]=>
+  int(1)
+  ["message"]=>
+  string(46) "All of the required rules must pass for author"
+}
+array(2) {
+  ["level"]=>
+  int(2)
+  ["message"]=>
+  string(30) "author must be of type integer"
+}
+array(2) {
+  ["level"]=>
+  int(2)
+  ["message"]=>
+  string(41) "author must have a length between 1 and 2"
+}
+array(2) {
+  ["level"]=>
+  int(0)
+  ["message"]=>
+  string(30) "Attribute user must be present"
+}