The commit to allow symlinks in pegasus-transfer broke PFN fall through

My fix to allow symlinking in pegasus-transfer broke the PFN priority fall through mechanism. An ln -s -f always works, even if the target file doesn't exist. Therefore, if the file:// PFN is missing, the symlink does not correctly fall through to the next transfer mechanism for the input data.

The fix is to test for the existence of the source file before calling ln. Then the fall though to another mechanism will work.

Pull request at https://github.com/pegasus-isi/pegasus/pull/9