[Debug] fix and enhance exception messages
bug #11759 [Debug] fix and enhance exception messages (nicolas-grekas)

This PR was merged into the 2.6-dev branch.

Discussion
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[Debug] fix and enhance exception messages

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11341
| License       | MIT
| Doc PR        | none

Commits
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8df1ce8 [Debug] fix and enhance exception messages