refactor: Consolidate dfx-core identity errors into error/identity.rs

[ericswanson-dfinity]

Description

There isn't really any benefit to having a bunch of tiny files each of which defiles a single error. This PR moves all error types from dfx-core/errors/identity/*.rs to dfx-core/errors/identity.rs. This PR makes no changes to the error types themselves, only moves them.

I'll be following up with changes that split up FsError into per-method error types, which will add some new error types, so I wanted to do this first.

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.