Only require the user to input the passphrase once, unless creating
a wallet.
Should they mis-enter the passphrase, they will be warned Electrum
couldn't pair the device, and when they actually need to use it
they will be prompted again.
Fixes#1672
Tell the user and ask if they want to try again. If they
say no, raise a silent exception. Apply this more friendly
behaviour to the install wizard too (see issue #1668).
ThomasV
Neil Booth