[Debug] Fixed FlattenException lacks access to original Exception

`FlattenException` lacks access to the original `Exception`, preventing access to e.g. `ErrorException::getSeverity()`.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -