Make generation of png images for pdf pages more resiliant.

We now use ghostscript to output pngs directly from the original pdf,
rather than using convert on the split pages. This should make corrupt
or strange pdfs less likely to cause issues.

As well, if a pdf page conversion fails (for any reason, including that
the original pdf is missing...) it will be logged, and a blank page
generated, and processing will continue.
Merge pull request #270 from kepstin/presentation-resiliance

Make generation of png images for pdf pages more resiliant.