How to get the original entered password value on registration form error?
You could always use the Craft request object to repopulate the fields, however I don't think it's common practice to repopulate password fields for security reasons.

Another thing that is considered bad practice is differentiating behaviour for partially correct credentials - the response from the web server should always be the same whether one part or multiple parts are missing from a correct authorization submission to prevent information disclosure to a potential attacker.