Completion endpoint answers 500 to data of another shape

Closes #3638136.

The completion endpoint checks the shape of the decoded body before anything is changed and answers 400 with a message that names the wrong part (e.g. Invalid data: the entry of course 12 is not an object.), instead of a TypeError and 500. The empty body of Moodle's connection test keeps its answer.

• MoodleCompletionRestResource::invalidData(): an object of course entries, each an object; total a number; users an object of objects.
• Unit test CompletionDataTest covers the accepted shapes and every refused one.
• Tested against a site with the REST resource: {} → 200, {"12":1}, {"userid":1,"courseid":1}, {"12":{"users":7}}, "x" → 400, a valid entry → 200.

The fix is one commit of its own, the CHANGELOG entry a second one.