screencastService: Signal errors via the internal dbus interface

Make sure gnome-shell gets notified of errors that happen during screen
recording using the screencastService, so that it can properly notify the
user about the error and tear down its state, too.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2752>

data/dbus-interfaces/org.gnome.Shell.Screencast.xml

@@ -92,5 +92,9 @@
       <arg type="b" direction="out" name="success"/>
     </method>
 
+    <signal name="Error">
+      <arg type="s" name="message"/>
+    </signal>
+
   </interface>
 </node>