Avoid incomplete cache dir interfering with subsequent runs

[j-atkins]

When running in --difficulty-mode easy the cache dir is not necessary. It only interferes with any subsequent runs and prompts the user to delete (which is confusing if then moving on to medium or hard when the cache should be ignored by the user).

Should probably be addressed at the point at which it throws an error (i.e. within _unique_id in run.py)