exception: Let MediaWiki.php control final output for ErrorPageError

exception: Let MediaWiki.php control final output for ErrorPageError

The same way it does already for non-error output. This makes
it so that doPreOutputCommit() consistently happens between
the staging of output and the actual sending of output.

It is still allowed for code to bypass this, such as for fatal
errors and for handlers that disable OutputPage (like Special:Export).

But for cases where we do want to perform doPreOutputCommit(), it
should be run consistently between staging and sending so that it
can make appropiate decisions based on the current state of
OutputPage.

Previously, the state of OutputPage seen by doPreOutputCommit()
would be the broken/incomplete output of a seemingly succesful
(possibly cacheable) user action, which would then, after
doPreOutputCommit() runs, be completely replaced by $e->report()/
$out->showErrorPage().

This is a prerequisite for being able to reliably send cookie-block
cookies on error pages (next patch).

Bug: T233594
Change-Id: Iaeaf5e55a5868e6be534ddda73f3b56b9d6ef8f0