Add a better error message when cloudpickle is missing (#4342)
Add a better error message when cloudpickle is missing (#4342)


jcrist/dask,blaze/dask,blaze/dask,ContinuumIO/dask,dask/dask,mrocklin/dask,jcrist/dask,ContinuumIO/dask,dask/dask,mrocklin/dask,jakirkham/dask,jakirkham/dask