[BUGFIX] Use named export of BackendExceptionModule in FormEngine

The BackendException class is exported as named (not as default)
export in BackendException.ts, and therefore the module consumer
needs to access the BackendException property of the module.
Fix FormEngine to do so.

Releases: master, 10.4
Resolves: #92249
Change-Id: Id416e7ee4753db033301c43482b786c38075e13a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65688


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>