Make polygonize operation interruptable

git-svn-id: http://svn.osgeo.org/geos/branches/3.4@3979 5242fede-7e19-0410-aef8-94bd7d2200fb
Polygonizer: do not pretend all exceptions are due to edge invalidity

git-svn-id: http://svn.osgeo.org/geos/branches/3.4@3980 5242fede-7e19-0410-aef8-94bd7d2200fb