CCX: Conflicting start and end dates do not show an error

[carsongee]

Tested on sandbox2o.

I noticed that conflicting start and end dates do not provide an error message and can go unnoticed. Changes can be saved as well. I'm not sure if an error message is desired. I provided two examples of conflicting dates below.

When a course is scheduled in studio, conflicting dates will produce an error message and changes cannot be saved.

However, individual subsections can be scheduled with conflicting dates and changes can be saved.

[pdpinch]

@amir-qayyum-khan I've verified https://github.com/edx/edx-platform/issues/10538 and it addresses this issue in the sidebar. But I think it still exists in the main schedule tree. Steps to reproduce:

1. Add a Chapter to a CCX.
2. In the schedule tree, click on the start date. Set it to be a date in the future.
3. For the same chapter, set the due date to be in the past.

Expected behavior:

An error message, like "Due date cannot be before start date.'

Actual behavior:

Date is accepted and can also be saved.

Work on this should probably wait for the schedule.js refactor, #146