In certain cases, montage code does not generate an output file even though the job itself succeeds.

Because of this the stage out jobs fail as the input does not exist ( as they are never created in the first place )

None of the underlying transfer tools we use ( like guc etc ) support the functionality of transferring as many files as possible. They fail on
first error. That makes it difficult to incorporate the behaviour in pegasus-transfer,
unless each transfer is invoked separately internally.

However we probably should consider doing this only( separate invocation of underlying transfer tool ) for the optional files

Another issue in this case would be that we could detect an error but there is no clear way to detect whether it
was because of a missing source file, unless there are specific exitcodes for this case in GUC and other tools we use in pegasus-transfer

For this there is a placeholder in the DAX to mark file transfer as optional.
We need to come up with a way to let pegasus-transfer know that a particular file transfer is optional in the input file passed to it