Improve "illegal path references in fixed output derivation" error (backport #12356)

[mergify]

The main improvement is that the new message gives an example of a path that is referenced, which should make it easier to track down. While there, I also clarified the wording, saying exactly why the paths in question were illegal.

By @bmillwood in #11673

Motivation

Context

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This is an automatic backport of pull request #12356 done by [Mergify](https://mergify.com).

[mergify]

This pull request has been removed from the queue for the following reason: checks failed.

The merge conditions cannot be satisfied due to failing checks:

• ❌ vm_tests

You should look at the reason for the failure and decide if the pull request needs to be fixed or if you want to requeue it.

If you want to requeue this pull request, you need to post a comment with the text: @mergifyio requeue

[Artturin]

Vm fail seems transient

vm-test-run-github-flakes> client # curl: (60) SSL certificate problem: unable to get local issuer certificate
vm-test-run-github-flakes> client # More details here: https://curl.se/docs/sslcerts.html
vm-test-run-github-flakes> client # 
vm-test-run-github-flakes> client # curl failed to verify the legitimacy of the server and therefore could not
vm-test-run-github-flakes> client # establish a secure connection to it. To learn more about this situation and
vm-test-run-github-flakes> client # how to fix it, please visit the web page mentioned above.

[roberth]

@Mergifyio queue

[mergify]

queue

🛑 The pull request has been removed from the queue default

The merge conditions cannot be satisfied due to failing checks.

You can take a look at Queue: Embarked in merge queue check runs for more details.

In case of a failure due to a flaky test, you should first retrigger the CI.
Then, re-embark the pull request into the merge queue by posting the comment
@mergifyio refresh on the pull request.

[roberth]

@Mergifyio refresh

[mergify]

refresh

✅ Pull request refreshed