core: be strict when parsing connection in AddAndActivateConnection

core: be strict when parsing connection in AddAndActivateConnection

AddAndActivateConnection is allowed to provide an incomplete connection
that will be completed by NetworkManager. That is, a connection that
does not verify.

But we still want to catch invalid properties or unknown setting types.
Thus, we want to reject invalid partial connections.

This possibly rejects invalid requests from clients that were accepted
before. Thus this change has the potential to break misbehaving clients.