Catch cuFFT error code

At the moment the code does only report an error code, like

error: cufftPlan3d(*plan, *nx, *ny, *nz, (cufftType)*type) failed with error 5

We should return a cleaner error message, see the relevant enums: https://docs.nvidia.com/cuda/cufft/index.html#return-value-cufftresult