app: Use 'g_message' (like the help plug-in) when GVFSd-HTTP is not present

Closes #13553

Such issue tends to affect only AppImage users (e.g. when running it in
KDE installations). This is because AppImage files are not ran by
any host helper (e.g. snapd or flatpak) to connect to GVFSd and is
not possible to bundle GVFSd due to its own nature (it is daemon).

Since GVFS is listed in the INSTALL file as a required dependency, it is
fair enough to be more clear when it is not present, otherwise users will
have outdated GIMP without knowing (like happened in macOS in the past).

We are already doing this with GIMP help plug-in by the way. The
difference is that, at GIMP start, such techinical error message can
be surprising, but even so it is needed as feedback for fail on the
update check button in the end of day. Let's follow GIMP help on this.

app/gimp-update.c

@@ -414,8 +414,22 @@ gimp_check_updates_callback (GObject      *source,
     {
       gchar *uri = g_file_get_uri (G_FILE (source));
 
+#ifndef GIMP_CONSOLE_COMPILATION
+#ifndef G_OS_WIN32
+      g_message ("%s: loading of %s failed: %s\n\n%s",
+                 G_STRFUNC, uri, error->message,
+                 _("Perhaps you are missing GIO backends and need "
+                   "to install GVFS?"));
+#else
+      g_message ("%s: loading of %s failed: %s\n\n%s",
+                 G_STRFUNC, uri, error->message,
+                 _("Perhaps you are missing GIO backends."));
+#endif /* G_OS_WIN32 */
+    
+#else
       g_printerr ("%s: loading of %s failed: %s\n", G_STRFUNC,
                   uri, error->message);
+#endif /* GIMP_CONSOLE_COMPILATION */
 
       g_free (uri);
       g_clear_error (&error);