Enhance BokehJS error reporting

[bryevdv]

Following on previous work, some additional possible improvements:

• larger target to "close"
• collapsible stack trace detail
• friendlier and more informative wording.

This would be my ideal:

Bokeh has encountered the following error:                                     close [x]

xr is null

<click to expand full stack trace>

[pepijndevos]

Looks like this issue has quite a history! Would this cover Python exceptions, or is it purely about JS exceptions? Seems like JS exceptions should pretty much be Bokeh bugs, but Python exceptions could be quite common, and I don't see a good way to present them to the user currently. Seems like the app just inexplicably stops working, with a stacktrace on the servers side.