Pass obvious MHS failure messages back to Gen2

When a PFS command sent from gen2 fails, we throw a g2cam CamCommandException, whose content gets displayed to the observer.

We currently just pass back a bland message about which command failed. And have in the past discussed enforcing elaborate conventions for labelling failure messages inside the MHS world.

But actually, real failures almost all come with a directly associated and available failure message. If such a thing exists, pass that back to Gen2 instead of the current message.