isl::stat::error indicates a run-time error. We should not abuse it to
enforce an early abort of the foreach call. We should soon see a new
isl_*_every function family, which allows for specific return values to
abort early. When these are available, we should introduce these
deliberately to indicate early aborts.
In same cases we also abused these return values to indicate if a
certain value has been found or not. To avoid this abuse an extra
boolean is introduced to track this state.