Use finally instead of try-catch-throw

Use finally instead of try-catch-throw

When this exception handler was introduced in change Ifbc1db4b82 (commit
9636d44448), MediaWiki still supported PHP 5.3.2, where finally was not
available. It was added in PHP 5.5, though, which we’ve required for a
while now, so we can simplify this a bit.

Change-Id: I7c967fb713de7495739e18ff6c6058a630129fda