Including the rest of the config block that the errors occurs in for debugging trace for invalid extends

Problem to solve

The debugging trace for invalid extends is not semantically useful enough. Currently, it requires the user to go back and dig through the raw YAML file for a line number (if gitlab-ce#51846 is implemented) in order to find out what is wrong exactly.

Intended users

Further details

This has been scoped of from https://gitlab.com/gitlab-org/gitlab-ce/issues/51846 and can be considered a follow up.

Proposal

The debugging trace for invalid extends could be made more informative by including the rest of the config block that the errors occurs in. If combined together with https://gitlab.com/gitlab-org/gitlab-ce/issues/62930 and the format/layout of the config block makes sense, this will make a solid improvement over gitlab-ce#51846

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

Links / references