gdm: Handle absence of Fprint.Manager service

We rely on the service to detect whether a fingerprint reader is
present. It is fine to not support fingerprint authentication
when the service is missing, but currently we don't handle this
case at all and end up with a non-functional login screen.

https://bugzilla.gnome.org/show_bug.cgi?id=780063

js/gdm/fingerprint.js

@@ -23,6 +23,12 @@ function FprintManager() {
                                    g_object_path: '/net/reactivated/Fprint/Manager',
                                    g_flags: (Gio.DBusProxyFlags.DO_NOT_LOAD_PROPERTIES) });
 
-    self.init(null);
+    try {
+        self.init(null);
+    } catch(e) {
+        log('Failed to connect to Fprint service: ' + e.message);
+        return null;
+    }
+
     return self;
 }