That is, when `latexdiff-so` can be found, use it instead of `latexdiff`
Fixes#27.
The latexdiff manual recommends the standalone version:
> Because latexdiff uses internal functions of Algorithm:Diff whose
> calling format or availability can change without notice, the
> preferred method is now to use the standalone version.
I also added `set -o pipefail` for better erroring.