Don't show errors loading draft

This particularly prevents the initial error all users will see when the
app first loads.

TODO: implement something more elegant.

src/window.vala

@@ -83,7 +83,7 @@ public class WriteAs.MainWindow : Gtk.ApplicationWindow {
         init_folder();
         try {
             open_file(draft_file());
-        } catch (Error err) {canvas.buffer.text = err.message;}
+        } catch (Error err) {}
         restore_styles();
 
         set_default_size(800, 600);