fold: reword error message for public commits
This rewords the error message when attempting to fold public csets.
It is somewhat clearer to not use contractions in formal writing. It
also uses i18n for it. This commit also adds a new test for this error
message.
all: tutorials/tutorial.rst static/logo-evolve.ico
sphinx-build . ../html/
tutorials/tutorial.rst: tutorials/tutorial.t
python test2rst.py tutorials/
static/logo-evolve.ico: static/logo-evolve.svg
convert -resize 36x36 static/logo-evolve.svg static/logo-evolve.ico