trezor: more user friendly when cannot connect

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).

lib/wizard.py

@@ -36,9 +36,6 @@ MSG_RESTORE_PASSPHRASE = \
       "Note this is NOT a password.  Enter nothing if you did not use "
       "one or are unsure.")
 
-class UserCancelled(Exception):
-    pass
-
 class WizardBase(PrintError):
     '''Base class for gui-specific install wizards.'''
     user_actions = ('create', 'restore')