Fix getPageDimensions failure handling

getPageDimensions returning false for failure wasn't being handled
properly, causing ugly output. The doc comment on that method was
wrong as well.

Most notably causing random whitespace output:
https://commons.wikimedia.org/wiki/?curid=22151015
(screenshot: http://i.imgur.com/c21EpVx.png).

Bug: 41281
Change-Id: I1a49474309e15808928f877dfc29ae366d028928
Merge "Fix getPageDimensions failure handling"