Show SlowDown error message if backend is busy (#7521)

or if there are too many open file descriptors.

cmd/object-api-errors.go

@@ -57,6 +57,8 @@ func toObjectErr(err error, params ...string) error {
 		}
 	case errDiskFull:
 		err = StorageFull{}
+	case errTooManyOpenFiles:
+		err = SlowDown{}
 	case errFileAccessDenied:
 		if len(params) >= 2 {
 			err = PrefixAccessDenied{
@@ -137,6 +139,13 @@ func (e StorageFull) Error() string {
 	return "Storage reached its minimum free disk threshold."
 }
 
+// SlowDown  too many file descriptors open or backend busy .
+type SlowDown struct{}
+
+func (e SlowDown) Error() string {
+	return "Please reduce your request rate"
+}
+
 // InsufficientReadQuorum storage cannot satisfy quorum for read operation.
 type InsufficientReadQuorum struct{}