Throw error if CUDA-aware MPI is not found for distributed GPU architecture

[glwagner]

cc @simone-silvestri

[simone-silvestri]

nice

[glwagner]

Hmm, we need to take care because apparently only openmpi allows us to check this:

help?> MPI.has_cuda
  MPI.has_cuda()

  Check if the MPI implementation is known to have CUDA support. Currently only Open MPI provides a mechanism to check, so it will return false with other implementations (unless overridden). For "IBMSpectrumMPI" it will
  return true.

  This can be overridden by setting the JULIA_MPI_HAS_CUDA environment variable to true or false.

  │ Note
  │
  │  For OpenMPI or OpenMPI-based implementations you first need to call Init().

  See also MPI.has_rocm for ROCm support.