[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/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py