fix(FAN-41): catch the EnrollmentNotAllowed and InvalidEnrollmentAttribute

[MaferMazu]

Description

This PR catches EnrollmentNotAllowed and InvalidEnrollmentAttribute exceptions in favor of returning a status 400 bad request. This fixes the 500 error when you make a post request to the /api/enrollment/v1/enrollment.

How to test

• Have an environment with this version of edx-platform.
• Have the filter configured and a course with the proper "other course settings" as we explained in the FAN-41 PR.
• With a user without an allowed domain, make a post request to /api/enrollment/v1/enrollment with {"course_details":{"course_id": "<your_course_id>"}}, and you are going to get an error 400 instead of 500.

Before

Response in /api/enrollment/v1/enrollment

Logs

After

Response in /api/enrollment/v1/enrollment

Logs

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[felipemontoya]

This looks exactly as we discussed in the call.
The code is tested in a development environment and the screenshots are pasted here. I think we are good to merge. Just waiting for Ivo's approval