Fix cluster verify with simulate-errors

In simulate errors mode, the test "ntime_diff is not None" will be
ignored, and thus a None value will try to be formatted as %.01f. We
workaround this by formatting it before, and then only using %s, which
can format a 'None' value.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>