[edd_receipt] - Display/load an error template

I didn't find a way to style or customize the 'Sorry, trouble retrieving payment receipt.' error message in the receipt shortcut.
It is customizable through the shortcode [edd_receipt error='custom message'] but I think thats not dynamic enough.

So this pull-request is a proposal on how to display a (customizable) template 'shortcode-receipt-error.php' instead of just returning the plain error message.

It doesn't effect the default behavior in any way (it still just displays the error message), but leaves the option to design and style the error page - e.g. explain to the user why this error might have occured and give him some options on what to do next.