Better jinja errors

[plaindocs]

@ericholscher @mxsasha Can we add a file and line number to these errors?

Handler <function render_rst_with_jinja at 0x7f08a08b6b00> for event 'source-read' threw an exception (exception: 'unconf' is undefined)

[ericholscher]

Maybe. It's almost certainly coming from the conf.py, but guessing it's run in some undefined context.

[plaindocs]

Most likely to be in load_conference_context_from_yaml in _ext/core.py after @mxsasha 's refactor a while back?

[mxsasha]

Do you have an example of a build that failed on this? Then I can mess around with it and see if we can print more context. Looked through some recent failures but did not see this type.

[plaindocs]

I thought it was one of the recent broken ones in the Australia PR, but all I'm getting now is this

Handler <function render_rst_with_jinja at 0x7fc53fd82b00> for event 'source-read' threw an exception (exception: include/schedule2022.rst)

in 0908bf2

[mxsasha]

I can't reproduce that last one, as the branch is gone.

For the initial error about unconf being undefined: #1833. Line numbers are not available in the error.
The root cause here is that our YAML validator is quite flexible, to allow historical YAML files too. We could make a separate schema for newer files, or just make a stricter schema and only validate newer files, as the old ones don't really change.