When running `make epub`, Make would try to satisfy the target
'src/plfa/acknowledgements_epub.md' by calling run-liquid.lua. But due
to the use of shell redirection, Make would always create the file
acknowledgements_epub.md, even if run-liquid.lua failed, in which case
acknowledgements_epub.md would be empty. This empty file would still
satisfy the 'src/plfa/acknowledgements_epub.md' target, allowing the
pandoc command line in the 'out/plfa.epub' target to proceed---and later
fail.
To avoid this, we now (a) add a bunch of error checking to run-liquid.lua,
and (b) write directly to acknowledgements_epub.md instead of relying on
shell redirection, so that acknowledgements_epub.md will only be created
(or modified) if run-liquid.lua succeeds.
Use liquid-lua to generate acknowledgements_epub.md, which is used by
pandoc to build the EPUB book. Also stop using the '{%-' / '-%}'
syntax, which liquid-lua does not support, in templates.
Also add a vertical space between the first bullet in each bulleted
list so pandoc recognizes bulleted lists as such. This has no effect on
the the website.
