Direct users to add a grading form if it doesn't exist

[djarran]

Describe the bug
Not a bug, but an edge case caused by user misconfiguration that could be handled to inform users.

While testing, some assignments were using the 'Rubric' grading method without having set an advanced grading form. However, the option to view the breakdown report the plugin provides is still shown. Since no grading form was created/assigned to the assignment, the get_grading_definition function in locallib.php errors when executed.

To Reproduce
Steps to reproduce the behavior:

1. Create new course
2. Add an assignment to module
3. Set grading method to 'Rubric' or 'Marking Guide'
4. Option to view breakdown report is shown
5. Clicking this option displays error

Expected behavior
Direct and notify users to create an advanced grading form.

Screenshots

Desktop (please complete the following information):

• OS: Ubuntu 22.04
• Browser: Firefox 120.0