Don't return resources

[davidbrochart]

This led to the following error when a cell was erroring out:

jinja2.exceptions.UndefinedError: 'tuple object' has no attribute 'get'

[maartenbreddels]

I'd like to have a test for this, do you feel like doing that?

[davidbrochart]

It's tricky because a test with e.g. a syntax error in a cell doesn't fail, and doesn't show the error in stdout or stderr as if Voila were run on the CLI.

[maartenbreddels]

It should give this error msg in red right? Run with --debug .... I think that feature isn't really tested at all.

[davidbrochart]

We want to test the presence of 'tuple object' has no attribute 'get', but it's not there with pytest.

[maartenbreddels]

I think we don't have to test that detailed. By default, voila should render a clean error message, now it does not do that. So I think if we just add a new test notebook, with say an import error, or syntax error, we can test if the rendered HTML contains that nice error message. What do you think about that?

[davidbrochart]

Sounds good, I added a test for that and checked that it was failing before this PR.