fix bug

If the 'regular expression' or 'replace expression' changes fleetly with
'live update' checked, RegexReplacer may works out an old result related
to the old 'regular expression' or 'replace expression', but treats that
result as the latest result.
fix bug

'stopCurrentWorking' may cause new calculation be cancelled, so create a
new method 'fireCalculationFailed' to avoid that problem.