Proper error handling for missing OpenMM state variables

[salotz]

When originally written I didn't know how (or wasn't possible) to cleanly handle errors from OpenMM when retrieving state information that wasn't available for the state, e.g. velocities, forces, etc. typically.

It looks like now these all raise OpenMMException types so this should be able to handled better now.

We may want to rework this being a warning entirely, but minimally it can raise a better warning that includes the OpenMMException message and error type.