modelize: broken mproperties can have definitions

[privat]

so, when keep-going is activated, mproperty.intro and other related services wont crashes. Thus fixes #2599 moreover, the mpropdefs attempts are registered in the model (if one need them?).

This might however add some unexpected effects (one does not simply add broken entities into mordor^Wmodel).

TODO: the same thing with classes?

[privat]

test this please

[privat]

test this please

[privat]

test this please

[Morriar]

That's one of the problem with is_borken, you never know what you can do with them before an assert fails.

Exceptions could be useful here!