Improve messages when submissions change status around the time limit

[gkreitz]

New messages mostly follows the discussion in #384. Gets rid of the special case for partially_accepted, we now give a message or warning if a submission changes status within [time_limit / ac_to_time_limit, time_limit * time_limit_to_tle].

Some examples (ignore directories and the actual time limits, intentionally weird to trigger messages):

ERROR partially_accepted/sol.py (Python 3 (w/PyPy3)) got TLE [CPU: 0.06s @ testcase secret/subtask2/01], expected AC
   partially_accepted/sol.py (Python 3 (w/PyPy3)) would get AC (50) with time limit >= 0.04s
ERROR wrong_answer/js.py (Python 3 (w/PyPy3)) got TLE [CPU: 0.04s @ testcase sample/1], expected WA
   wrong_answer/js.py (Python 3 (w/PyPy3)) is sensitive to time limit. 0.01s -> TLE, [time limit 0.03s], 0.03s -> AC (50), 0.04s -> AC (100)
WARNING accepted/js.py (Python 3 (w/PyPy3)) is within ac_to_time_limit (2) of time limit 0.05s. It takes 0.04s to get AC (100).
   accepted/js.py (Python 3 (w/PyPy3)) OK: AC (100) [CPU: 0.04s @ testcase secret/subtask2/06]

Fixes #384

[gkreitz]

I think the improved wording here mostly resolves #414 too. I'll set this as closing that too.