Return 404 if course not found and 403 for permission issues and update tests

[Imran92]

Related to #4959

Changes proposed in this Pull Request

Previously we were returning a 403 from the course bulk actions API if the user did not have permission to edit the course or if the course was not found. Now we'll send a 404 if the course is not found.

Testing instructions

• Create a few courses
• Create a few students
• Create a few normal posts
• Add some students to a few courses using this API Add REST API for adding students to courses #4968
• Try hitting the API with post ids or post ids mixed with course ids
• Try hitting the API with random ids which do not match with any course or post
• Try hitting the API without nonce and cookies
• Try the same for this API as well Api for removing multiple users from multiple courses #4976
• Observe that you receive 403s and 404s properly

Here's a sample request exported from vscode's thunder client. You can import this to your client and change the URL, Cookie, Nonce, Course Ids and Student Ids to check easily
thunder-collection_Sensei.json.zip

[merkushin]

Looks good!
I only want to suggest slightly improve tests with the new error code.

[merkushin]

I'd like to see also check of the new error code (sensei_course_student_batch_action_missing_course) in tests for 404.
Maybe something like that:

	public function testAddUsersToCourses_CourseNotFoundGiven_ReturnsCourseNotFoundResponse() {
		// ...
		$expected = [
			'status_code' => 404,
			'error_code' => 'sensei_course_student_batch_action_missing_course',
		];
		self::assertSame( $expected, $this->exportErrorResponse( $response ) );
	}

	private function exportErrorResponse( $response ): array {
		return [
			'status_code' => $response->get_status(),
			'error_code' => $response->get_data()['code'] ?? null,
		];
	}

[Imran92]

Thank for the suggestion @merkushin :) I have updated the tests to assert with error code as well. Can you kindly take a look again?

[merkushin]

👍