Title: Improve RaceOk error format for array impl

Body: Duplicate changes from https://github.com/yoshuawuyts/futures-concurrency/pull/94/files for RaceOk impl

src/future/race_ok/array/error.rs

@@ -1,5 +1,6 @@
 use core::fmt;
 use core::ops::{Deref, DerefMut};
+use std::error::Error;
 
 /// A collection of errors.
 #[repr(transparent)]
@@ -13,19 +14,26 @@ impl<E, const N: usize> AggregateError<E, N> {
     }
 }
 
-impl<E: fmt::Debug, const N: usize> fmt::Debug for AggregateError<E, N> {
+impl<E: Error, const N: usize> fmt::Debug for AggregateError<E, N> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let mut list = f.debug_list();
-        for err in self.inner.as_ref() {
-            list.entry(err);
+        writeln!(f, "{self}:")?;
+
+        for (i, err) in self.inner.iter().enumerate() {
+            writeln!(f, "- Error {}: {err}", i + 1)?;
+            let mut source = err.source();
+            while let Some(err) = source {
+                writeln!(f, "  ↳ Caused by: {err}")?;
+                source = err.source();
+            }
         }
-        list.finish()
+
+        Ok(())
     }
 }
 
-impl<E: fmt::Debug, const N: usize> fmt::Display for AggregateError<E, N> {
+impl<E: Error, const N: usize> fmt::Display for AggregateError<E, N> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(self, f)
+        write!(f, "{} errors occured", self.inner.len())
     }
 }
 
@@ -43,4 +51,4 @@ impl<E, const N: usize> DerefMut for AggregateError<E, N> {
     }
 }
 
-impl<E: fmt::Debug, const N: usize> std::error::Error for AggregateError<E, N> {}
+impl<E: Error, const N: usize> std::error::Error for AggregateError<E, N> {}