[web/views] Fix exception in CWProperty edit form (closes #5087412) Also add missing help message. The overall display is ugly, but at least the message is shown.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl