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

Author: bshaffer

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

@@ -194,7 +194,7 @@ public static function checkBool(&$var)
     public static function checkMessage(&$var, $klass)
     {
         if (!$var instanceof $klass && !is_null($var)) {
-            throw new \Exception("Expect message.");
+            throw new \Exception("Expect $klass.");
         }
     }