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.

(As a somewhat amusing side-effect, this *increases* the quality of graphics in the slides, because it doesn't recompress embedded jpg images at lower quality like the page-splitting does)