(bug 37627) generic exception for not logged in users

(bug 37627) generic exception for not logged in users

We have various place in MediaWiki core and in extensions which are
showing anonymous user a very standard error page about them not being
logged in. Each developer ends up writing its own because we do not
provide a generic error, that is what this patch does.

This UserNotLoggedIn exception, when called, will show the usual
ErrorPage with a default title and default reason text. That makes it as
easy to use as doing:

 if( $user->isAnon() ) {
 	throw new UserNotLoggedIn();
 }

One can override the default reason by passing a message key as the
first parameter:

 if( $user->isAnon() ) {
 	throw new UserNotLoggedIn( 'nologin-reason-text' );
 }

In that case, the page title will still be the default 'Not Logged In.'

Change-Id: Id81272995627bf0f5bbef785230a8e6e4e8582ca