several improvements to watchfolder.py.
several improvements to watchfolder.py.

- robust handling of user aborts, exception and rendering errors.
- removed .appleseed intermediate extension from output (image) filenames.

gospodnetic/appleseed,appleseedhq/appleseed,pjessesco/appleseed,est77/appleseed,Biart95/appleseed,est77/appleseed,est77/appleseed,aytekaman/appleseed,gospodnetic/appleseed,Vertexwahn/appleseed,luisbarrancos/appleseed,aytekaman/appleseed,est77/appleseed,aiivashchenko/appleseed,appleseedhq/appleseed,Aakash1312/appleseed,luisbarrancos/appleseed,aiivashchenko/appleseed,pjessesco/appleseed,dictoon/appleseed,Vertexwahn/appleseed,docwhite/appleseed,glebmish/appleseed,luisbarrancos/appleseed,appleseedhq/appleseed,pjessesco/appleseed,Biart95/appleseed,dictoon/appleseed,Vertexwahn/appleseed,Aakash1312/appleseed,dictoon/appleseed,Biart95/appleseed,Aakash1312/appleseed,aiivashchenko/appleseed,luisbarrancos/appleseed,gospodnetic/appleseed,Aakash1312/appleseed,Aakash1312/appleseed,docwhite/appleseed,aiivashchenko/appleseed,aytekaman/appleseed,gospodnetic/appleseed,appleseedhq/appleseed,aytekaman/appleseed,appleseedhq/appleseed,docwhite/appleseed,Vertexwahn/appleseed,gospodnetic/appleseed,glebmish/appleseed,glebmish/appleseed,Biart95/appleseed,aiivashchenko/appleseed,aytekaman/appleseed,est77/appleseed,Biart95/appleseed,glebmish/appleseed,luisbarrancos/appleseed,pjessesco/appleseed,Vertexwahn/appleseed,dictoon/appleseed,docwhite/appleseed,dictoon/appleseed,pjessesco/appleseed,glebmish/appleseed,docwhite/appleseed