box/lua: don't mask error object methods by payload fields

With commit eb2c6a4a ("box/lua: allow to set custom error payload
fields") it is possible to add a field named "raise", but it will
override the `error_object:raise()' method. Forbid it. The user can
still access masked payload fields through the `unpack()' method.

NO_DOC=unreleased
NO_CHANGELOG=unreleased