[CWL] Allow "keep:" references on command line.
arvados-cwl-runner lets you provide input parameter values on the command line. However, currently it assumes they are relative paths local files and doesn't allow you to use references to keep. This should be fixed to permit references to keep.
As a secondary problem, "file not found" when cehcking command line parameters results in a backtrace, rather being caught and printing a friendlier error message.