refactor: Split up FsError into method-specific error types (partial)

[ericswanson-dfinity]

Description

Move some of the error types out of FsError into method-specific error types. The goal is to remove FsError altogether.

How Has This Been Tested?

Refactor covered by existing tests

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [ ] I have edited the CHANGELOG accordingly.
• [ ] I have made corresponding changes to the documentation.