more explicit error message

[falkben]

"i should not get called" seems to be related to not having openmp installed. we should probably include that information.

#include <omp.h>
#else
void	omp_set_dynamic(int x){
	std::cout << "I should not get called\n";
}
void omp_set_num_threads(int x){
	std::cout << "I should not get called\n";
}
#endif

https://github.com/neurodata/packedForest/blob/master/src/baseFunctions/fpForest.h#L10-L18