Experimental

Regardless of the reason, the solver did in fact not come up with a solution. We don't know why, maybe it is unsatisfiable, or the solver cannot determine if it is satisfiable (e.g. Cassowary doesn't say a non-linear constraint is unsatisfiable - it just doesn't know). I think CouldNotSatisfy just says that the solver did not find a solution, for whatever reason. The original, solver specific exception should be available on the CouldNotSatisfy error object.