[BUGFIX] RTE Linking with file upload fails when no folder is selected

Solution: Catch exceptions raised when the potential target folder or
the default user upload folder does not exist.

Resolves: #48512
Releases: 6.2
Change-Id: Ic0218f218b045562d0ea5499ac0440848ddf3028
Reviewed-on: https://review.typo3.org/27323
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland