includes the expected class in the exception, otherwise the error is harder to track down (#3371)

php/src/Google/Protobuf/Internal/GPBUtil.php

diff --git a/php/src/Google/Protobuf/Internal/GPBUtil.php b/php/src/Google/Protobuf/Internal/GPBUtil.php
index 76f84cb..05d5c39 100644
--- a/php/src/Google/Protobuf/Internal/GPBUtil.php
+++ b/php/src/Google/Protobuf/Internal/GPBUtil.php
@@ -194,7 +194,7 @@
     public static function checkMessage(&$var, $klass)
     {
         if (!$var instanceof $klass && !is_null($var)) {
-            throw new \Exception("Expect message.");
+            throw new \Exception("Expect $klass.");
         }
     }