fix bug in file deletion in CERN stageout plugin

The last commit introduced a bug in the CERN stageout plugin that
made file deletions fail. Fixed that and also restructured the
DeleteMgr class to not use and exception handler as a flow
control mechanism. Also added a printout so some possible
error messages aren't supressed anymore.
Merge pull request #4412 from hufnagel/improvedeleteerror

fix bug in file deletion in CERN stageout plugin