Improved exception text in distributor_factory.py a little bit.

8 jobs for master
Status Job ID Name Coverage
  Test
passed #145960
test_minimal

00:00:56

passed #145961
test_mpi

00:01:52

passed #145962
test_mpi_fftw

00:01:41

passed #145963
test_mpi_fftw_hdf5

00:02:43

canceled #145838
test_minimal

canceled #145839
test_mpi

canceled #145840
test_mpi_fftw

canceled #145841
test_mpi_fftw_hdf5