Document w/ swagger UI: POST /assessment/{id}/submit responds 400 if has null answers

[ning-y]

For a student, if an assessment has answers that are null (i.e., nothing saved for these answers), POST /assessment/{id}/submit responds with status code 400.

I think this is good behaviour, but also I think it should be documented in the swagger UI because it's a potential gotcha.

[ning-y]

Also, POST /assessment/{id}/submit responds with 404 if a student has not yet attempted the assessment (i.e. status === :not_attempted). This makes perfect sense wrt to the backend implementation, but not really to a frontend dev.

[ning-y]

(just putting this out there for next gen CP3108 people)

[tuesmiddt]

Ah shit the 404 doesn't make sense. I'll change that when I need a mental break.