Rollup merge of #142216 - nealsid:refcell-logging, r=tgross35

Miscellaneous RefCell cleanups

- Clarify `RefCell` error messages when borrow rules are broken
- Remove `Debug` impl for `BorrowError`/`BorrowMutError` since `#derive(Debug)` provides identical functionality
- Rename `BorrowFlag` to `BorrowCounter`

Author: workingjubilee