Handle failure task_output that won't fit in the model column.

common/lib/capa/capa/capa_problem.py

common/lib/capa/capa/tests/test_responsetypes.py

common/lib/xmodule/xmodule/capa_module.py

lms/djangoapps/instructor_task/api_helper.py

lms/djangoapps/instructor_task/models.py

lms/djangoapps/instructor_task/tasks.py

lms/djangoapps/instructor_task/tasks_helper.py

lms/djangoapps/instructor_task/tests/test_api.py

lms/djangoapps/instructor_task/tests/test_tasks.py

lms/djangoapps/instructor_task/views.py