✅ Add new exceptions

tests/ExceptionTest.php

@@ -0,0 +1,29 @@
+<?php
+
+use PHPUnit\Framework\TestCase;
+use Shikiryu\SRSS\Exception\ChannelNotFoundInRSSException;
+use Shikiryu\SRSS\Exception\PropertyNotFoundException;
+use Shikiryu\SRSS\Exception\UnreadableRSSException;
+use Shikiryu\SRSS\SRSS;
+
+class ExceptionTest extends TestCase
+{
+    public function testPropertyNotFound()
+    {
+        $srss = new SRSS();
+        $this->expectException(PropertyNotFoundException::class);
+        $srss->notfound = 'true';
+    }
+
+    public function testRssNotFound()
+    {
+        $this->expectException(UnreadableRSSException::class);
+        $rss = SRSS::read('not_found.xml');
+    }
+
+    public function testMissingChannel()
+    {
+        $this->expectException(ChannelNotFoundInRSSException::class);
+        $rss = SRSS::read(__DIR__ . '/resources/invalid-no-channel.xml');
+    }
+}