Added error message for when target folder was removed

Whent trying to upload/rename/create files in a folder that was removed
or rename, the correct error message is now shown.

In the case of upload of multiple files, the upload is cancelled.

This situation can happen if the target folder was renamed or removed
from another browser window or client.

Fixes #6871 

Please review/test @DeepDiver1975 @karlitschek @schiesbn @ser72