Filter error handler scores before rendering view statistics page

[damianhxy]

Description

Filter out grader_ids of -1 before rendering the view statistics page.

Motivation and Context

In #1643, scores are marked with a grader_id of -1 if an error occurs (for context, see the PR).

However, this led to an issue in viewing the view statistics page if any such scores exist.

This is since in grading.rb, we map the grader_ids with :find_user, which only handles grader_id = 0 specially. As a result, an error occurs on the line @course.course_user_data.find(i) if a grader_id of -1 is present.

How Has This Been Tested?

First, make a submission that triggers the error handler. e.g. create an assessment whose autograder does not output a score json at the end (e.g. install hello.tar, modify src/driver.sh, run make). Then submit to it.

Afterwards, access the view statistics page.

BEFORE (Error)

AFTER (Renders)

Types of changes

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

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting

[lykimchee]

Tested with a different autograder (not hello.tar):

In master:

In hide-error-handler-statistics:

lgtm! :>