Add error checking to ControlledMovie.link_additional

If something goes wrong, we can skip setting up the controls.  Note that
other parts of the pipeline are not error checked.
set window roles
Merge pull request #41 from rschroll/movie-error

Add error checking to ControlledMovie.link_additional
Merge pull request #46 from tgbit/master

set window roles
Merge branch 'master' of https://github.com/davvil/pdfpc