This is the second half of T155053. Basically, the 404 error page in mobile front end has a very short HTML where the `<html>` tag lacks the 'lang' and 'dir' attributes, and the HTML also lacks the `<meta charset...>` tag. These should be added so that the 404 message is well-formed and shown correctly in RTL wikis.
This is because it invokes a 404 using wfHttpError
This should be updated to do things the MediaWiki way™
= Acceptance criteria
 Use $this->getOutput()->setStatusCode( 404 );
 Use $this->getOutput()->addHTML
 Show a less generic error on bad contributions link. Suggested text below
= Suggested error text for contributions 404
 Heading: Bad username given
 Description: Cannot look for contributions without a user or with a user that does not exist.