Conditions

Every condition cl-sl signals derives from sl-error, so a caller can catch all of them with one handler-case clause.

• sl-error -- base condition.
• invalid-dimensions (invalid-dimensions-width, invalid-dimensions-height) -- signaled by make-world or world-resize given a non-positive width or height.
• unknown-variant (unknown-variant-name) -- signaled by make-train for a variant keyword not in +train-variants+ (:normal, :little, :fly).