Stop pipeline if bokeh server does not initialize

[kvgc]

Hi,
The pipeline continues to run all the reduction steps even if the bokeh server fails to start. It would be useful to stop the pipeline if the bokeh server fails to initialize. And likewise, as noted in #63 , the bokeh server does not seem to close if the pipeline is stopped abruptly.
kvgc

[MNBrod]

I'll add this as a default option for the next release, thanks!