Check for bad results returned from Asymptote server

[sean-fitzpatrick]

Currently, if compilation of an Asymptote figure fails on the University of Alberta Asymptote server, the pretext script doesn't notice. We could ask John Bowman if he's able to implement better handling of compilation errors server side, but I feel like he's already doing us a big favour by making the server available.

What happens now is that the server returns a file (HTML, or PDF, depending on which format we're building), but the file is not valid HTML/PDF. If the pretext script were able to run some sort of validation on the files as they came in, we'd know there's a problem much sooner in the build process. Otherwise, we don't find out until the HTML build fails (or LaTeX compilation fails).