Implement Error for OccupiedError.

library/alloc/src/collections/btree/map/entry.rs

@@ -93,6 +93,19 @@ impl<K: Debug + Ord, V: Debug> Debug for OccupiedError<'_, K, V> {
     }
 }
 
+#[unstable(feature = "map_try_insert", issue = "none")]
+impl<'a, K: Debug + Ord, V: Debug> fmt::Display for OccupiedError<'a, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "failed to insert {:?}, key {:?} already exists with value {:?}",
+            self.value,
+            self.entry.key(),
+            self.entry.get(),
+        )
+    }
+}
+
 impl<'a, K: Ord, V> Entry<'a, K, V> {
     /// Ensures a value is in the entry by inserting the default if empty, and returns
     /// a mutable reference to the value in the entry.