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.

syntax: re
/figures/[^/]+\.png$
^docs/build/
^docs/html/
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
\.err$
^tests/easy_run.sh$
^build/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$