[native] Fix media-cache "directory not found" warnings

Summary:
Fixes ENG-3620.

When clearing cache, removing an already removed directory would cause a warning to be logged. Also fixed one more place where a similiar warn could appear.

Test Plan:
The easiest way to test this is to use the iOS Simulator:

• Console-logged cache directory and opened its parent dir in Finder. Manually added some content there (it's also possible to simply open encrypted media).
• Logged-out - observed the directory content being removed.
• Logged in again. Manually deleted the dir in finder.
• Logged out again - empty dir was created.

Reviewers: ashoat, atul, kamil

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7583