Check for missing pdftoppm depending on how poppler-utils is installed

When I upgraded to poppler 0.24.5, pdftoppm was not compiled because the
script had --disable-splash-output set for some reason. It looks like this is a
new-ish option to disable a significant part of poppler for people concerned
about such things.

This change just updates the error message to suggest what the problem 
might be. Check for it last, after confirming that other poppler utils are present.

For OS X Homebrew the solution is:
brew uninstall poppler
brew install poppler --with-splash-output