Add error handling to Students modal

[donnapep]

Changes proposed in this Pull Request

Adds an error message if something goes wrong with adding, removing, or resetting progress.

Testing instructions

To force errors to happen, I changed the following lines to return 404 instead of WP_HTTP::OK:

• https://github.com/Automattic/sensei/blob/feature/student-management/includes/rest-api/class-sensei-rest-api-course-progress-controller.php#L86
• https://github.com/Automattic/sensei/blob/feature/student-management/includes/rest-api/class-sensei-rest-api-course-students-controller.php#L90
• https://github.com/Automattic/sensei/blob/feature/student-management/includes/rest-api/class-sensei-rest-api-course-students-controller.php#L117

Then:

• Select Add to Course from the dropdown, select a course, and click the Add to Course button.
• Ensure the message "Unable to add student. Please try again." is displayed.
• Close the modal.
• Select Remove from Course from the dropdown, select a course, and click the Remove from Course button.
• Ensure the message "Unable to remove student. Please try again." is displayed.
• Close the modal.
• Select Reset or Remove Progress from the dropdown, select a course, and click the Reset or Remove Progress button.
• Ensure the message "Unable to reset or remove progress for this student. Please try again." is displayed.
• Close the modal.

Screenshot / Video

Add

Remove

Reset or Remove Progress

[donnapep]

Note that the test that is failing was not introduced in this PR, but was added at some point when adding tests for the modal. We'll still need to look into this separately.

[merkushin]

Looks good and works well 👍

As for tests, locally all tests passed.