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
Ideally this would be an option in ErrorPageError (perhaps even the default),
but its constructor isn't very suitable for that.
After this lands, uses of ErrorPageError should be audited to see if it makes
sense to emit a 400 status code.
Change-Id: I4beb6a4f256446b98b66d5e4bcdbab8f247441a8
