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).

plugins/trezor/clientbase.py

@@ -1,7 +1,7 @@
 from sys import stderr
 
 from electrum.i18n import _
-from electrum.util import PrintError, SilentException
+from electrum.util import PrintError, UserCancelled
 
 
 class GuiMixin(object):
@@ -27,7 +27,7 @@ class GuiMixin(object):
         # gets old very quickly, so we suppress those.
         if msg.code in (self.types.Failure_PinCancelled,
                         self.types.Failure_ActionCancelled):
-            raise SilentException()
+            raise UserCancelled()
         raise RuntimeError(msg.message)
 
     def callback_ButtonRequest(self, msg):

plugins/trezor/plugin.py

@@ -220,8 +220,6 @@ class TrezorCompatiblePlugin(HW_PluginBase):
         process.  Then create the wallet accounts.'''
         devmgr = self.device_manager()
         device_info = devmgr.select_device(wallet, self)
-        if not device_info:
-            raise RuntimeError(_("No devices found"))
         devmgr.pair_wallet(wallet, device_info.device.id_)
         if device_info.initialized:
             task = partial(wallet.create_hd_account, None)

plugins/trezor/qt_generic.py

@@ -11,9 +11,8 @@ from ..hw_wallet.qt import QtHandlerBase
 
 from electrum.i18n import _
 from electrum.plugins import hook, DeviceMgr
-from electrum.util import PrintError
+from electrum.util import PrintError, UserCancelled
 from electrum.wallet import Wallet, BIP44_Wallet
-from electrum.wizard import UserCancelled
 
 PASSPHRASE_HELP_SHORT =_(
     "Passphrases allow you to access new wallets, each "
@@ -317,10 +316,7 @@ def qt_plugin_class(base_plugin_class):
         device_id = self.device_manager().wallet_id(window.wallet)
         if not device_id:
             info = self.device_manager().select_device(window.wallet, self)
-            if info:
-                device_id = info.device.id_
-            else:
-                window.wallet.handler.show_error(_("No devices found"))
+            device_id = info.device.id_
         return device_id
 
     def query_choice(self, window, msg, choices):