add rerun_on_exception kwarg to async scheduler
add rerun_on_exception kwarg to async scheduler

This reruns the failing task in the main process when an exception
occurs.  This enables the use of tools like ``pdb`` on parallel code
with errors.

jayhetee/dask,clarkfitzg/dask,mikegraham/dask,mrocklin/dask,mraspaud/dask,jakirkham/dask,wiso/dask,blaze/dask,mrocklin/dask,jayhetee/dask,blaze/dask,ssanderson/dask,PhE/dask,jcrist/dask,vikhyat/dask,cpcloud/dask,clarkfitzg/dask,wiso/dask,jakirkham/dask,ContinuumIO/dask,chrisbarber/dask,mraspaud/dask,jcrist/dask,vikhyat/dask,PhE/dask,dask/dask,simudream/dask,ContinuumIO/dask,pombredanne/dask,gameduell/dask,cowlicks/dask,pombredanne/dask,dask/dask,simudream/dask,ssanderson/dask