Description:
------------
The MessageFormatter constructor returns null when using an invalid format string.

Reproduce code:
---------------
<?php
$example = new MessageFormatter('en_US', '{0,oops}');
var_dump($example);
?>

Expected result:
----------------
I expected to receive a MessageFormatter object. The output of the script above should be:

object(MessageFormatter)#1 (0) {
}

An invalid token in the format string should be ignored, possibly with an a error triggered, and a valid object returned. 

Alternately, an InvalidArgumentException exception could be thrown.

Actual result:
--------------
NULL