[BUGFIX] Exception\ParseError extends an inexistent class

The leading backslash in the base class name is missing.

Change-Id: I12e497ed78b2696a84bf678ed0153e2b4b627668
Resolves: #58221
Releases: 6.2
Reviewed-on: https://review.typo3.org/29672
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen