Fixed PathListingWidget errors when "/" path is invalid.
Fixed PathListingWidget errors when "/" path is invalid.

Invalid root paths occur quite frequently in the SceneHierarchy editor when an invalid filename has been entered in a SceneReader or AlembicReader. Those nodes will still display an error, but the SceneHierarchy now won't output a confusing stacktrace which distracts people from the root cause.

Fixes #528.

paulondc/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,cedriclaunay/gaffer,cedriclaunay/gaffer,cedriclaunay/gaffer,chippey/gaffer,GafferHQ/gaffer,davidsminor/gaffer,hradec/gaffer,boberfly/gaffer,davidsminor/gaffer,davidsminor/gaffer,boberfly/gaffer,goddardl/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,goddardl/gaffer,davidsminor/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,hradec/gaffer,johnhaddon/gaffer,paulondc/gaffer,goddardl/gaffer,johnhaddon/gaffer,goddardl/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,hradec/gaffer,chippey/gaffer,paulondc/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,chippey/gaffer,hradec/gaffer,hradec/gaffer,cedriclaunay/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,boberfly/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,paulondc/gaffer,andrewkaufman/gaffer,boberfly/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,chippey/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,johnhaddon/gaffer