shell: Use g_printerr instead of g_warning

These are really runtime errors, not warnings, so just use g_printerr.

origin commit:
https://git.gnome.org/browse/evince/commit/?id=adbed8b

shell/ev-application.c

@@ -285,7 +285,7 @@ ev_spawn (const char     *uri,
 		g_object_unref (ctx);
 	}
 	if (error != NULL) {
-		g_warning ("Error launching atril %s: %s\n", uri, error->message);
+		g_printerr ("Error launching atril %s: %s\n", uri, error->message);
 		g_error_free (error);
 	}
 
@@ -398,7 +398,7 @@ on_register_uri_cb (GObject      *source_object,
 
 	value = g_dbus_connection_call_finish (connection, res, &error);
 	if (!value) {
-		g_warning ("Error registering document: %s\n", error->message);
+		g_printerr ("Error registering document: %s\n", error->message);
 		g_error_free (error);
 
 		_ev_application_open_uri_at_dest (application,
@@ -585,7 +585,7 @@ ev_application_unregister_uri (EvApplication *application,
 		NULL,
 		&error);
         if (value == NULL) {
-		g_warning ("Error unregistering document: %s\n", error->message);
+		g_printerr ("Error unregistering document: %s\n", error->message);
 		g_error_free (error);
 	} else {
                 g_variant_unref (value);