fix: stop get_course_users swallowing roster parse errors

[dave-doty]

Problem

Account.get_course_users wraps its scrape in:

except Exception:
    return None

Its annotated return type is list[Member], so on any parse failure a caller sees only:

TypeError: 'NoneType' object is not iterable

at their own for member in members loop, with nothing pointing at the real cause. I hit this on a course roster and it took a while to work out that the error was in the scrape, not the loop.

Cause

Three spots in get_course_members assume every roster row is a student. On a course with instructors, TAs or readers, each raises and is then swallowed:

line	assumption	raises
data_url.split("user_id=")	data-url is present whenever the js-rosterName button is	AttributeError
int(cells[num_submissions_column].text)	submissions cell is always numeric	ValueError
id_to_role[...]	data-role is always in {0,1,2,3}	KeyError

The first is the narrowest: the button is already guarded for None, but the attribute read .get("data-url", None) is not, so a name button without that attribute goes straight into .split().

The second and third are the common ones — a blank submissions cell for anyone who cannot submit takes down the entire roster, not just that row.

Fix

• get_course_users no longer swallows exceptions, matching its declared return type. Genuine failures (auth, 404, markup changes) now surface where they happen.
• user_id stays None when there is no data-url, which is what the field's own comment already documents: "only present for 'student' accounts in a course".
• a non-numeric submissions cell becomes 0.
• an unrecognised role id passes through as the raw id.

Each now degrades for the single member it concerns instead of discarding every member.

Tests

tests/test_course_members.py parses fixture HTML directly, so unlike the rest of the suite it needs no GRADESCOPE_CI_* credentials and runs in CI as-is. It covers a student row, a staff row with a blank submissions cell, a name button with no data-url, an unrecognised role id, and a mixed roster.

Against main four of the five fail (ValueError, AttributeError, KeyError); all five pass with this change. The student-row case passes both ways, confirming student parsing is unchanged.

I could not run the credentialed suite, having no CI accounts — it reports 33 failed / 3 passed identically on main and on this branch, every failure being ValueError: Invalid credentials.

[dave-doty]

For the record, Claude Opus wrote these fixes, and I didn't inspect them carefully, but they do appear to fix the error I was encountering.