DelayedFormat panics on to_string()

[korest]

I'm trying to format a DateTime with a pattern (possibly invalid 2f for millis, 3f and 6f work), however it panics on to_string:

fn main() {
   let dt = chrono::Local::now();
   let formatted = dt.format("%H:%M:%S%.2f");
   println!("{}", formatted.to_string());
}

result of a cargo run:

rust_test % cargo run 
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `target/debug/rust_test`
thread 'main' panicked at 'a Display implementation returned an error unexpectedly: Error', /rustc/574b64a97f52162f965bc201e47f0af8279ca65d/library/alloc/src/string.rs:2537:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

cargo 1.68.0-nightly (2381cbdb4 2022-12-23)
chrono v0.4.23

[djc]

Yeah, only 3, 6, 9 or f may follow a %. in a format string, so it panics. See #902 for a more robust error handling strategy.