Merge pull request #3948 from minrk/sphinx-dir

Sphinx and PDF tweaks for nbconvert:

- add `--interaction=batchmode` to pdflatex call to avoid waiting for interactive input.

- use sphinx.package_dir to find texinputs directory regardless of on-disk location.

Closes #3894.
log fatal error when PDF conversion fails

don't user batch mode, which suppresses too much output