Rollup merge of #40117 - SimonSapin:to-err-is-for-the-formatter, r=alexcrichton

Panic on errors in `format!` or `<T: Display>::to_string`

… instead of silently ignoring a result.

`fmt::Write for String` never returns `Err`, so implementations of `Display` (or other traits of that family) never should either.

Fixes #40103

diff --cc src/libcollections/string.rs
Simple merge