Prevent errors when xenvg is broken cluster verify

When vg_name is not returned at all, we currently abort with an internal
error. This is because we don't catch KeyError.

This patch adds a custom message for this case, and also adds KeyError
to the list of catched exceptions, just for safety.

On the other hand, we could also just remove this piece of code since
it's not used at all the ["dfree"] value.

Reviewed-by: ultrotter