[Bug-XBoard] Make non supported variant for engine non fatal

[Joshua Pettus]

Anyway we can make the non supported variant error for an engine, non fatal for 
xboard?  It’s rather easy to do, when you mean to switch the variant, but can’t 
until you switch the engine.  Or if you select an engine that is set to use one 
variant, but the second engine, that was previously loaded, doesn’t support it. 
I don’t really know much about it, but what if it just unloaded the engine if 
it doesn’t support the variant, rather then crash?

Thanks,
Josh