[Debug] fix error message on double exception
bug #12553 [Debug] fix error message on double exception (nicolas-grekas)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #12553).

Discussion
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[Debug] fix error message on double exception

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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1ed7606 [Debug] fix error message on double exception