If pdf build fails, dump .tex file into log directory

[rhinopotamus]

Say I'm building to pdf, but something is wrong with the generated LaTeX file. The LaTeX engine will give me some useful error messages with line numbers, but those point to the generated LaTeX file rather than the PreTeXt source.

That LaTeX file lives in some temp directory whose address is logged (I'm pretty sure this happens here), but it'd be nice if I could just look in the log directory (or maybe even the output directory?) for it.