Sisysphus: fixed edge case in delete-assignment.
Sisysphus: fixed edge case in delete-assignment.

* If the assignment's submission directory didn't exist for some
  reason it would explode, now it logs a warning and continues.

ucrcsedept/galah,brownhead/galah,galah-group/galah,brownhead/galah,galah-group/galah,galah-group/galah,ucrcsedept/galah,brownhead/galah,ucrcsedept/galah