In Zola templates that extend another template, variables declared at top-level are silently ignored

[LunarEclipse363]

Bug Report

I felt this deserves a new issue that's specifically focused on the UX problems rather than trying to get the one I made previously edited and/or reopened.

Environment and steps to reproduce

See #2485

Expected Behavior

Zola exits with an error, explaining that you cannot declare variables outside of blocks when extending another template.

Current Behavior

The variable declarations are silently ignored, producing either confusing error messages (that tell you nothing about the root cause of the problem) or unexpected output.

[Keats]

We could add an error for that in the next version of Tera but that's not a Zola issue itself.

[LunarEclipse363]

Sounds good, I think this issue should be kept around until a fix makes its way to Zola though.