Document and raise better exceptions about backend loading

[relativistic]

When I try to use the %%output cell magic to switch to the bokeh backend, I get the error "Error: list index out of range".

[jlstevens]

I believe you just need to load the bokehJS. At the top of the notebook, try:

import holoviews as hv
hv.notebook_extension('matplotlib', 'bokeh')

Then it should work as long as bokeh is installed.

One thing we should do is improve the error message here so thanks for reporting this!

[philippjfr]

Correct, definitely needs a better exception though. There's a few issues like this now that the plotting backends aren't imported by default, so we should catalogue them and make sure to raise better exceptions.

[relativistic]

Ah, yes, that fixed it. Thanks!

It might also be a good idea to be explicit this is required "Bokeh Backend" page of the docs, where this is described. I had assumed from the language there that you could use the cell output magic after only loading the matplotlib option in the hv.notebook_extension. Probably obvious to those who are more familiar with the way this kind of thing works, but it wasn't for me.

[philippjfr]

Reopening and repurposing this issue.

[jlstevens]

Should be easy enough to fix so I'm assigning this as something to fix by 1.7.

[philippjfr]

The output magic error message was improved, closing.

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