fix: surface EnrollmentNotAllowed message to instructor dashboard

[ccantillo]

fix: surface EnrollmentNotAllowed message to instructor dashboard

Problem

When a pipeline filter blocks enrollment (e.g. FilterEnrollmentRequireNIF), the specific error message was never shown to the instructor. Instead, a generic error was returned with no explanation.

Two cases were broken in process_single_student_enrollment:

1. Registered user blocked by a filter — enroll_email() raises EnrollmentNotAllowed with a descriptive message, which was caught by the broad except Exception handler and replaced with a generic "Something went wrong" message.

2. Unregistered email blocked by a pre_save signal guard — a ValidationError raised during enrollment was caught by the same except ValidationError used for email format validation, returning invalidIdentifier: True and "Invalid email address" — completely misleading.

Solution

• Separated validate_email into its own try/except block so email format errors are handled independently from errors raised during enrollment.
• Added except EnrollmentNotAllowed as exc to capture filter-blocked enrollments and surface str(exc) in the response.
• Added except ValidationError as exc for ValidationError raised during the enrollment process itself, surfacing exc.messages[0] instead of a hardcoded message.

Testing

1. Set up a course with a pipeline filter that blocks enrollment (e.g. a NIF requirement filter).
2. Try to enroll a registered user who does not meet the filter requirement via the instructor dashboard.
3. Verify the instructor sees the specific message from the filter instead of a generic error.
4. Try to enroll an unregistered email on the same course.
5. Verify the instructor sees the specific message from the pre_save guard instead of "Invalid email address".

[openedx-webhooks]

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