Improve code clarity and error messages for css selection

cms/djangoapps/contentstore/features/advanced-settings.py

cms/djangoapps/contentstore/features/common.py

cms/djangoapps/contentstore/features/course-team.py

cms/djangoapps/contentstore/features/grading.py

cms/djangoapps/contentstore/features/problem-editor.py

common/djangoapps/terrain/browser.py

common/djangoapps/terrain/ui_helpers.py

lms/djangoapps/courseware/features/navigation.py

lms/djangoapps/courseware/features/problems_setup.py