function getRollbackFunc(step) {
  switch(step.type) {
    case SAVE: return rollbackSave;
    case UPDATE:
    case REMOVE: return rollbackRemoveOrUpdate;
    case FILE_SAVE: return rollbackFileSave;
    case FILE_REMOVE: return rollbackFileRemove;
  }
}
Gets the correct rollback function for a step

@param step the step to rollback

@returns a function to rollback step