Check for result size failure in ApiQuery

If someone sets $wgAPIMaxResultSize crazily small and then queries a
large number of pages, it can cause ApiQuery to fail to populate the
'pages' node. Then submodules might add data to the node, leading to
errors from the XML formatter (or bogus output from other formatters).

While that particular failure mode is probably going away soon, it's
still good to catch the error situation.

Bug: 68989
Change-Id: If0b2eda421fd86dd7acc4fe48cec47741e214221