Use message instead of error when one seed failed

Use the message channel instead of the error message channel when one seed
failed. Otherwise in vessel view the cli might get written as failed even
if one of the seed actually succeeded. (Slicer considers that any message
written to cerr means the cli failed.)
Add spatial object nodes merging logic

This allow the user to reload a spatial object in another one without
re-writing it. The spatial object is simply added to the other.
Merge pull request #313 from vovythevov/InteractiveSegmentTubes-Fixes

Interactive segment tubes fixes