def: 6407
Added 'optional' flag to operator definition. When set the , upon execution failure the operator will copy the source to the output, rather than fail and halt the flavor execution

git-svn-id: svn+ssh://kelev.kaltura.com/usr/local/kalsource/backend/server/trunk/core@92434 6b8eccd3-e8c5-4e7d-8186-e12b5326b719