More "relpath" try/except protection...

Note: import gimp image also would need it, but it would anyway need a good cleanup (handling of paths is ugly), so did not bothered with it for now.

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4679 96b784f8-4ae3-dd11-aff7-0030488c597c