Give warning when markup outside a define block is found (thus ignoring base template)

[anthonyfok]

See discussion at #4808, where @GregoryVigoTorres wrote:

I also think it would be nice if an error got raised when markup outside a define block is found.

Maybe something like this?

WARNING: Content found outside of {{ define }} block(s) in layouts/index.html, ignoring base template layouts/_default/baseof.html

Speaking as a casual contributor to Hugo, I do not yet know if Go raises such a warning/error so that Hugo can expose it. I will investigate to satisfy my own curiosity. ;-)

[stale[bot]]

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[pedrodude]

Nothing to add aside from +1. I last used Hugo prior to the introduction of base templates and blocks and this was not at all obvious to me (as a "newbie") nor could I find any documentation reference, making this time-consuming to debug.