These three errors should be no problem. The first two are from packages (kupu and kss) that are no longer needed in Plone 4.3. They will be cleaned up during the upgrade. The third is probably fixable by installing the html2captioned utility. Ah, that is actually from the removed kupu. This might get cleaned up during the upgrade as well. But this is not what is causing the real problem.
You have tried both with and without the dry run option. To save the changes, you need to run it without dry run.
If there is a real problem, it should say something like 'the upgrade did not reach the latest version'.
I would then expect details to be visible in zeoclient.log or instance.log. Is anything there?
Also, an error may be visible when you scroll further down the page.