Previously, if an error was returned from the formatter that did not
originate in an underlying writer error, Write::write_fmt would return
successfully even if the formatting did not complete (was interrupted by
an `fmt::Error` return).
Now we choose to emit an io::Error with kind Other for formatter errors.
Since this may reveal error returns from `write!()` and similar that
previously passed silently, it's a kind of a [breaking-change].
