raise error for role API requests with nonexistent sections

[pretendWhale]

Motivation and Context

Currently, the roles API allows requests with non-existent or malformed section_name and sets the section to nil. This PR introduces an error message for this case.

Your Changes

Description:
Changed role API controller to raise an error if a section cannot be found based on the section_name parameter.

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)

Testing

Tested with the web api, and added tests for creating/updating with an invalid section name.

Questions and Comments (if applicable)

Checklist

• I have performed a self-review of my own code.
• I have verified that the pre-commit.ci checks have passed.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have added tests for my changes.

Pull request to make documentation changes (if applicable)

[coveralls]

Pull Request Test Coverage Report for Build 5490571978

• 34 of 34 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 91.621%

---

Files with Coverage Reduction	New Missed Lines	%
spec/models/course_spec.rb	2	99.21%

Totals
Change from base Build 5478301019:	0.01%
Covered Lines:	37095
Relevant Lines:	39943

---

💛 - Coveralls

[david-yz-liu]

@pretendWhale thanks! I realized as I was reviewing this that we do need a way for the user to remove a section from a student, e.g. with section_name: nil. This should be handled differently from either of the cases (1) section_name doesn't correspond to an existing section, and (2) section_name is not present in the params.

[pretendWhale]

@david-yz-liu Thanks for the catch, you're totally right!