Fix a bug in the GUI: not echoing error messages when tracking fails.

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
Fix two bugs in SparseFrameToFrameLinker.

- The graph was not instantiated.
- We were passing the wrong settings map.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>