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]

This issue has been automatically marked as stale because it has not had recent activity. The resources of the Hugo team are limited, and so we are asking for your help.
If this is a bug and you can still reproduce this error on the master branch, please reply with all of the information you have about it in order to keep the issue open.
If this is a feature request, and you feel that it is still relevant and valuable, please tell us why.
This issue will automatically be closed in the near future if no further activity occurs. Thank you for all your contributions.

[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.