[Debug] preserve modified error level
minor #10988 [Debug] preserve modified error level (nicolas-grekas)

This PR was merged into the 2.5 branch.

Discussion
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[Debug] preserve modified error level

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

Minor edge case, but still a bug fix.
Replaces https://github.com/symfony/symfony/pull/10978

Commits
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e40b717 [Debug] preserve modified error level