pbutils: discoverer: Set GError when NULL info is being returned.

When discovering the URI, if info is NULL, then instead of just returning NULL,
set the GError, so the error can be printed and notified.

https://bugzilla.gnome.org/show_bug.cgi?id=753701

gst-libs/gst/pbutils/gstdiscoverer.c

@@ -2107,6 +2107,9 @@ gst_discoverer_discover_uri (GstDiscoverer * discoverer, const gchar * uri,
   if (G_UNLIKELY (discoverer->priv->current_info)) {
     DISCO_UNLOCK (discoverer);
     GST_WARNING_OBJECT (discoverer, "Already handling a uri");
+    if (err)
+      *err = g_error_new (GST_CORE_ERROR, GST_CORE_ERROR_FAILED,
+          "Already handling a uri");
     return NULL;
   }