[BUGFIX] Import/Export Various Exceptions happening

Fixed exception shown when

1) uploading an existing file without "Overwrite" setting activated
2) opening export module as a non-admin
3) opening import module as a non-admin without permission
   to access an available file storage

Also improved error message for missing default file upload folder.

Resolves: #72765
Releases: master
Change-Id: Id1133d1a8adc6c28088ef85730fe01145b4fa3a2
Reviewed-on: https://review.typo3.org/45998


Reviewed-by: Martin Kutschker <martin.kutschker@ymail.com>
Tested-by: Martin Kutschker <martin.kutschker@ymail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>