Oembed warning if GD function is missing

I lost about 2 hours of my life debugging why a page was throwing a completely silent 500 error. Nothing in the logs and nothing on screen. It was because GD wasn't installed (yay me).

I've made changes so that there is a helpful error message if the GD function isn't available and removed the silencing of the errors from the function call (from the [PHP docs](http://www.php.net/manual/en/function.imagecreatefromstring.php)) I don't see any notice of errors/warnings/notices being thrown, so there is no point in silencing it - it simply makes debugging impossible.