revert last commit, wrong plugin

git-svn-id: http://svn.resourcespace.org/svn/resourcespace@5504 c08608d7-6e46-0410-86ca-f2a6f1370df5
FIX: fail less spectacularly in case of missing helper applications in image_processing; allows previews work to continue if things like text extraction fail.


git-svn-id: http://svn.resourcespace.org/svn/resourcespace@5505 c08608d7-6e46-0410-86ca-f2a6f1370df5