Bokeh dashboard (when explicitly requested) needs to either error or display port

I launch `dask-scheduler` as follows:

```
dask-scheduler --bokeh
```

and see

```
distributed.scheduler - INFO - -----------------------------------------------
distributed.scheduler - INFO - Clear task state
distributed.scheduler - INFO - Scheduler at: tcp://10.142.155.162:8786
distributed.scheduler - INFO - Local Directory: /tmp/scheduler-d8zsyupb
distributed.scheduler - INFO - -----------------------------------------------
```

Since I explicitly asked for the Bokeh console, it would be helpful if an error message could be displayed describing why it was not launched. Or was it launched? If so, I'd expect to see the port listed.

I have the same issue in the notebook: `client = Client(); client` displays the client information, but there is no mention of Bokeh, so I can only assume that it did not work.

```
In [5]: sys.version
Out[5]: '3.6.3 (default, Oct 3 2017, 21:16:13) \n[GCC 7.2.0]'

In [6]: dask.__version__
Out[6]: '0.17.5'

In [7]: distributed.__version__
Out[7]: '1.21.8'

In [8]: bokeh.__version__
Out[8]: '0.12.5'
```