Debug mode for handling undefined variables

[roed314]

Currently, if you're working on a template and you make an error in jinja where there is an undefined variable, it just won't display anything. This makes it difficult to find the problem. Jinja has a configuration option to make it raise an error. Roughly, we can enable this by adding the following to app.py:

if is_debug_mode():
    from jinja2 import StrictUndefined
    app.jinja_options["undefined"] = StrictUndefined

But this doesn't quite work for several reasons. It would be great to make it work.