rust: Cleanup error handling helpers for storage

* Rename the helper methods on `Error` to better convey their intent.
* Add `check_non_negative`.
* Improve doc comments to better explain how each is used.

Bug: None
Test: build.py --test com.android.trusty.rust.storage.test
Change-Id: I3d57be63d118312ddf5d4ab3dcdacab64cd8bbef
2 files changed