[BUGFIX] Exception thrown in update wizard if file does not exists

When fetching a file that does not exists a exception is
thrown in the update wizard to update RTE file links to FAL.
The exception will now be catched.

Resolves: #53127
Releases: 6.2
Change-Id: I81a204506abfe2f462142ccc504f3a4e2cef138f
Reviewed-on: https://review.typo3.org/25004
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters