Throw useful error if graphviz fails.
Throw useful error if graphviz fails.

The conda version of graphviz (the c library, not the python binding of
the same name) is missing png support. In this case, an empty string is
returned from the command. Now we throw a useful error in this case,
pointing the user to the relevant github issue.

jcrist/dask,dask/dask,mrocklin/dask,blaze/dask,mrocklin/dask,gameduell/dask,jcrist/dask,mikegraham/dask,mraspaud/dask,cpcloud/dask,jakirkham/dask,mraspaud/dask,ContinuumIO/dask,jakirkham/dask,blaze/dask,chrisbarber/dask,dask/dask,cowlicks/dask,ContinuumIO/dask