Print storage errors during distributed initialization (#6441)

This commit will print connection failures to other disks in other nodes
after 5 retries. It is useful for users to understand why the
distribued cluster fails to boot up.

cmd/posix.go

@@ -274,6 +274,10 @@ func (s *posix) String() string {
 	return s.diskPath
 }
 
+func (s *posix) LastError() error {
+	return nil
+}
+
 func (s *posix) Close() error {
 	close(s.stopUsageCh)
 	s.connected = false