Add fatal errors for VV and twin-range MTS

Michael never implemented the multiple-time stepping with the VV
integrator family and constraints (see code that calls
combine_forces() from update_coords() in src/mdlib/update.c). Probably
that means the multiple-time-step regime was not tested with VV
either. Strictly speaking, these new fatal errors have scope that is
wider than is clearly warranted, but it is not clear the
no-constraints VV path was only ever as bad as the broken leap-frog
path is (see #1400).

I suspect VV+constraints will work with the incoming fixes for
leap-frog, but until someone wants to use it (and why would they?),
then I'm not going to test that it works as well as it does with
leap-frog.

Change-Id: Ib61d0fb7661bca2101c04423a6af1744420c06ab