[BUGFIX] RTE Linking with file upload fails

[BUGFIX] RTE Linking with file upload fails

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

The change  https://review.typo3.org/27323 from ticket
http://forge.typo3.org/issues/48512 should be applied
in few places more.

Resolves: #56751
Related: #48512
Releases: 6.2
Change-Id: I7446697d64a580203f8f772d4bba690542dd7cb2
Reviewed-on: https://review.typo3.org/28247
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Frans Saris
Tested-by: Frans Saris