Don't crash if ":1.41 was not provided"

Relates to bug #1

openvpn-u2f-ask-password

@@ -31,7 +31,7 @@ try:
     # "ValueError: Namespace Notify not available for version 0.8"
     gi.require_version('Notify', '0.7')  # hide version warning
 
-    from gi.repository import Notify as GObjectNotify
+    from gi.repository import Notify as GObjectNotify, GLib
 
     # We expect python3-dbus through these dependent apt packages:
     #   ubuntu-server -> software-properties-common -> python3-dbus
@@ -149,7 +149,15 @@ class Notification:
 
     def show(self):
         if self._notification:
-            self._notification.show()
+            try:
+                self._notification.show()
+            except GLib.Error as e:
+                # (Observed during boot.)
+                # gi.repository.GLib.Error: g-dbus-error-quark:
+                #   GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown:
+                #   The name :1.41 was not provided by any .service files (2)
+                print('ERROR showing notification: {}'.format(e))
+
         print('> [{}]\n> {}'.format(self._title, self._body))