More informative grade status

As discussed in #456 , currently, `NONE` (in csv) or `- ∅ -` (in gradebook view) can mean:

1. a user didn't start a session (if this is a session related opportunity).
2. a user didn't submit a session
3. a user submitted a session but not graded
4. a user submitted a session and graded, but the percentage is not a `valid` one.

Can we differentiate those? We now have `- ∅ -`, `(exempt)`, float percentage, and `(other state)`, and can't cover the cases above. Or we will have to navigator to each user's single_gradebook to see what's happening when there were `NONE` and `- ∅ -` . Along with this, #263, #417, #430 also need to be worked round.