guard against missing bokeh library in dscheduler
guard against missing bokeh library in dscheduler

amosonn/distributed,broxtronix/distributed,broxtronix/distributed,dask/distributed,blaze/distributed,blaze/distributed,mrocklin/distributed,dask/distributed,amosonn/distributed,dask/distributed,mrocklin/distributed,mrocklin/distributed,amosonn/distributed,dask/distributed,broxtronix/distributed