Modified tex2pdf to fail without hanging

This can happen when files are missing. pdflatex asks for files via stdin, but
tex2pdf is capturing stdout to make up for pdflatex not providing a valid
integer returncode to indicate failure.
Reduced docutils halt_level from 4 to 3

I found that my documents that had errors consistently caused subsequent
processing by pdflatex to fail completely. This change makes documents with
ReST errors fail before being passed on to pdflatex.
Merge pull request #50 from jaberg/tex2pdf

Modify tex2pdf to fail without hanging.