Verify key passphrase

[kushaldas]

Right now if the user types wrong passphrase for the key while uploading the key to the card, it fails saying "Failed to upload to the card.". We should first verify if the passphrase is correct (or show proper error), and then try to upload to the card.

We can sign a small message to verify the passphrase.

This came up while initial testing by Harlo.