Leave password line edit enabled on errors

[fmoc]

Contributes to #9336.

New behavior:

I think we shouldn't disable the close button, because that'll bloat the (already relatively complex) code within the dialog, and requires the user to uncheck the checkbox first (in that case, we'd actually have to disable the close button generally when information is missing to make the UX consistent). Just removing the password and asking for a new one in the line edit combined with displaying the error message gives the user sufficient feedback to continue. Keeping the password makes little sense either, we should not store secrets for longer than necessary.

Upon success, a generic placeholder will be inserted in the line edit, as before.

[sonarcloud]

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0.0% Duplication

[TheOneRing]

Could you also check for the expiration date, I think it will also be disabled on invalid input (if it is possible to put in invalid values)

[fmoc]

The format is enforced by the picker. For instance, with the en locale, a short format mm/dd/YY is used. You cannot enter invalid characters, I haven't managed to generate an error message.

(By the way, I don't like abbreviated years in date specifications... perhaps we should change that in a future release.)