Check CUDA return codes in fft_scalar.cuFFT

#325 (closed) indicates missing stopper when GPU out-of-memory happens.

The return code of EVERY CUDA call must be checked.

With this fix.

 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     Error in routine  fft_scalar_cuFFT: cfft3d_gpu (2):
  cufftPlanMany failed
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

     stopping ...

Error code 2 returns

    CUFFT_ALLOC_FAILED   = 2,  //  cuFFT failed to allocate GPU or CPU memory

as documented in https://docs.nvidia.com/cuda/cufft/index.html#cufftresult