Trezor/KeepKey: force watching only improvements

Only warn about watching only once given a chance to pair.
Failure to pair makes watching-only and warns.
In error message to user, distinguish between failure to connect
and failure to pair.

gui/qt/main_window.py

@@ -311,6 +311,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         title = 'Electrum %s  -  %s' % (self.wallet.electrum_version,
                                         self.wallet.basename())
         if self.wallet.is_watching_only():
+            self.warn_if_watching_only()
             title += ' [%s]' % (_('watching only'))
         self.setWindowTitle(title)
         self.password_menu.setEnabled(self.wallet.can_change_password())